Undefined Behavior testing zstd in Gentoo

**Describe the bug**
A clear and concise description of what the bug is.

I was enabling UBSan in Gentoo, and it of course failed with UBSan errors.

**To Reproduce**
Steps to reproduce the behavior:
1. Turn on UBSan flags
2. emerge -av1 app-arch/zstd
3. Tests failed

**Expected behavior**
A clear and concise description of what you expected to happen.

Tests pass

**Screenshots and charts**
If applicable, add screenshots and charts to help explain your problem.

```
meson test --print-errorlogs --num-processes 16 --timeout-multiplier=2
ninja: Entering directory `/var/tmp/portage/app-arch/zstd-1.5.7-r1/work/zstd-1.5.7/build/meson-abi_x86_64.amd64'
ninja: no work to do.
1/10 zstd:test-invalidDictionaries OK 0.02s
2/10 zstd:test-poolTests OK 0.43s
3/10 zstd:test-longmatch OK 1.83s
4/10 zstd:test-fullbench-2 FAIL 27.53s exit status 1
>>> MSAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1:print_stacktrace=1 MALLOC_PERTURB_=224 MESON_TEST_ITERATION=1 /var/tmp/portage/app-arch/zstd-1.5.7-r1/work/zstd-1.5.7/build/meson-abi_x86_64.amd64/tests/fullbench -i1 -P0
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stderr:
*** Zstandard speed analyzer 1.5.7 64-bits, by Yann Collet (Sep 10 2026) ***
Sample 10000000 bytes :
0#compress : 2741.4 MB/s (10000241)
1#decompress : 5532.4 MB/s (10000000)
2#compress_freshCCtx : 2398.5 MB/s (10000241)
3#decompressDCtx : 3717.3 MB/s (10000000)
4#compressContinue : 2201.5 MB/s (10000241)
5#compressContinue_extDict : 1992.8 MB/s (10000223)
6#decompressContinue : 3029.8 MB/s (10000000)
7#compressStream : 2109.1 MB/s (10000237)
8#compressStream_freshCCtx : 1778.6 MB/s (10000237)
9#decompressStream : 3988.0 MB/s (10000000)
10#compress2 : 2042.0 MB/s (10000241)
11#compressStream2, end : 1892.9 MB/s (10000241)
12#compressStream2, end & short : 1772.1 MB/s (10000241)
13#compressStream2, continue : 2227.2 MB/s (10000237)
14#compressStream2, -T2, continue : 1653.3 MB/s (10000237)
15#compressStream2, -T2, end : 1417.4 MB/s (10000241)
16#compressSequences : 2650.6 MB/s (10000241)
ERROR: Scenario 17: This mode cannot generate an uncompressed block
../../programs/benchfn.c:116:13: runtime error: null pointer passed as argument 1, which is declared to never be null
#0 0x6350b8cd51be in BMK_benchFunction ../../programs/benchfn.c:116
#1 0x6350b8cfc9b7 in BMK_benchTimedFn ../../programs/benchfn.c:220
#2 0x6350b8cf3157 in benchMem ../../tests/fullbench.c:888
#3 0x6350b8cef061 in benchSample ../../tests/fullbench.c:957
#4 0x6350b8cef061 in main ../../tests/fullbench.c:1202
#5 0x714bafe8d3bd in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:59
#6 0x714bafe8d4da in __libc_start_main_impl ../csu/libc-start.c:360
#7 0x6350b8cef964 in _start (/var/tmp/portage/app-arch/zstd-1.5.7-r1/work/zstd-1.5.7/build/meson-abi_x86_64.amd64/tests/fullbench+0x22964) (BuildId: 086f15e8ca2807af53ef92323b30bad2c495e8f7)

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5/10 zstd:test-fullbench-1 FAIL 27.97s exit status 1
>>> MALLOC_PERTURB_=153 MSAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1:print_stacktrace=1 MESON_TEST_ITERATION=1 /var/tmp/portage/app-arch/zstd-1.5.7-r1/work/zstd-1.5.7/build/meson-abi_x86_64.amd64/tests/fullbench -i1
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stderr:
*** Zstandard speed analyzer 1.5.7 64-bits, by Yann Collet (Sep 10 2026) ***
Sample 10000000 bytes :
0#compress : 137.2 MB/s ( 3239077)
1#decompress : 757.8 MB/s (10000000)
2#compress_freshCCtx : 148.4 MB/s ( 3239077)
3#decompressDCtx : 844.6 MB/s (10000000)
4#compressContinue : 139.3 MB/s ( 3239077)
5#compressContinue_extDict : 134.4 MB/s ( 3239062)
6#decompressContinue : 597.1 MB/s (10000000)
7#compressStream : 120.5 MB/s ( 3239075)
8#compressStream_freshCCtx : 133.2 MB/s ( 3239075)
9#decompressStream : 704.9 MB/s (10000000)
10#compress2 : 137.4 MB/s ( 3239077)
11#compressStream2, end : 137.3 MB/s ( 3239077)
12#compressStream2, end & short : 121.9 MB/s ( 3239077)
13#compressStream2, continue : 146.8 MB/s ( 3239075)
14#compressStream2, -T2, continue : 243.5 MB/s ( 3240235)
15#compressStream2, -T2, end : 205.9 MB/s ( 3240239)
16#compressSequences : 211.4 MB/s ( 3239695)
17#compressSequencesAndLiterals : 208.1 MB/s ( 3239695)
../../programs/benchfn.c:116:13: runtime error: null pointer passed as argument 1, which is declared to never be null
#0 0x60e0e86161be in BMK_benchFunction ../../programs/benchfn.c:116
#1 0x60e0e863d9b7 in BMK_benchTimedFn ../../programs/benchfn.c:220
#2 0x60e0e8634157 in benchMem ../../tests/fullbench.c:888
#3 0x60e0e8630061 in benchSample ../../tests/fullbench.c:957
#4 0x60e0e8630061 in main ../../tests/fullbench.c:1202
#5 0x7280b09473bd in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:59
#6 0x7280b09474da in __libc_start_main_impl ../csu/libc-start.c:360
#7 0x60e0e8630964 in _start (/var/tmp/portage/app-arch/zstd-1.5.7-r1/work/zstd-1.5.7/build/meson-abi_x86_64.amd64/tests/fullbench+0x22964) (BuildId: 086f15e8ca2807af53ef92323b30bad2c495e8f7)

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6/10 zstd:test-decodecorpus OK 30.04s
7/10 fast - zstd:test-zstd-fast OK 41.01s
8/10 zstd:test-zstream-3 OK 90.06s
9/10 zstd:test-zstream-1 OK 102.78s
10/10 zstd:test-fuzzer OK 218.94s

Summary of Failures:

4/10 zstd:test-fullbench-2 FAIL 27.53s exit status 1
5/10 zstd:test-fullbench-1 FAIL 27.97s exit status 1

Ok: 8
Fail: 2

Full log written to /var/tmp/portage/app-arch/zstd-1.5.7-r1/work/zstd-1.5.7/build/meson-abi_x86_64.amd64/meson-logs/testlog.txt
```

**Desktop (please complete the following information):**
- OS: Gentoo Linux
- Version: 2.18
- Compiler: gcc
- Flags: -O2 -pipe -ggdb3 -fsanitize=undefined
- Other relevant hardware specs: 32GB RAM
- Build system: Meson

**Additional context**
Add any other context about the problem here.