pthreads app fails with assert in vmareas when run under drcachesim

**Describe the bug**
See initial post here https://groups.google.com/g/DynamoRIO-Users/c/kBWn4YVQRe0 and more details below

The following assert is seen when pthreads app is run under drcachesim. There is no error seen if run with just drrun.
```
$ bin32/drrun -debug -loglevel 4 -t drcachesim -- suite/tests/bin/pthreads.pthreads 10000

...

Estimation of pi is 3.141592654423132

```

We can see that there is no mmap2 syscall with size 0 in this case.
```
$ grep "^syscall: mmap2" logs/pthreads.pthreads.2463.00000000/log.0.2463.html
syscall: mmap2 addr=0x00000000 size=0x2000 prot=0x3 flags=0x22 offset=0x0 fd=-1
syscall: mmap2 addr=0x00000000 size=0x1a97b prot=0x1 flags=0x2 offset=0x0 fd=3
syscall: mmap2 addr=0x00000000 size=0x1f298 prot=0x5 flags=0x802 offset=0x0 fd=3
syscall: mmap2 addr=0xf7bb2000 size=0x2000 prot=0x3 flags=0x812 offset=0x1b fd=3
syscall: mmap2 addr=0xf7bb4000 size=0x1298 prot=0x3 flags=0x32 offset=0x0 fd=-1
syscall: mmap2 addr=0x00000000 size=0x1dba20 prot=0x5 flags=0x802 offset=0x0 fd=3
syscall: mmap2 addr=0xf7b90000 size=0x3000 prot=0x3 flags=0x812 offset=0x1d5 fd=3
syscall: mmap2 addr=0xf7b93000 size=0x2a20 prot=0x3 flags=0x32 offset=0x0 fd=-1
syscall: mmap2 addr=0x00000000 size=0x801000 prot=0x0 flags=0x20022 offset=0x0 fd=-1
syscall: mmap2 addr=0x00000000 size=0x801000 prot=0x0 flags=0x20022 offset=0x0 fd=-1

```
When I run the app under gdb(`gdb suite/tests/bin/pthreads.pthreads 10000`), I see that the last two syscalls come from allocate_stack in pthread_create

```
(gdb) where
#0 0xf7fd51b9 in __kernel_vsyscall ()
#1 0xf7eab464 in __GI___mmap (addr=0x0, len=0x801000, prot=0x0, flags=0x20022, fd=0xffffffff, offset=0x0)
at ../sysdeps/unix/sysv/linux/mmap.c:42
#2 0xf7f99e4d in allocate_stack (stack=, pdp=, attr=0xffffd37c) at allocatestack.c:563
#3 __pthread_create_2_1 (newthread=0xffffd3ec, attr=0x0, start_routine=0x56556228 , arg=0x565570f7) at pthread_create.c:644
#4 0x56556368 in main (argc=0x1, argv=0xffffd4b4)
at /home/prasun/dynamorio-debug-32b-failure/dynamorio/suite/tests/pthreads/pthreads.c:100

(gdb) where
#0 0xf7fd51b9 in __kernel_vsyscall ()
#1 0xf7eab464 in __GI___mmap (addr=0x0, len=0x801000, prot=0x0, flags=0x20022, fd=0xffffffff, offset=0x0)
at ../sysdeps/unix/sysv/linux/mmap.c:42
#2 0xf7f99e4d in allocate_stack (stack=, pdp=, attr=0xffffd37c) at allocatestack.c:563
#3 __pthread_create_2_1 (newthread=0xffffd3e8, attr=0x0, start_routine=0x56556228 , arg=0x565570f9) at pthread_create.c:644
#4 0x56556388 in main (argc=0x1, argv=0xffffd4b4)
at /home/prasun/dynamorio-debug-32b-failure/dynamorio/suite/tests/pthreads/pthreads.c:101
```

**To Reproduce**
See above

**Expected behavior**
No crash

**Screenshots or Pasted Text**
See above

**Versions**
- What version of DynamoRIO are you using?
- 7e5a3e84
- Does the latest build from https://github.com/DynamoRIO/dynamorio/releases solve the problem?
- No
- What operating system version are you running on? ("Windows 10" is *not* sufficient: give the release number.)
- Ubuntu 18
- Is your application 32-bit or 64-bit?
- 32 bit

**Additional context**