Uninits in RtlCaptureContext in many tests on Server2016

Extra uninits are present in many tests on Github Actions Server2016:
```
2020-12-07T02:55:07.8828198Z 4: Error #10: UNINITIALIZED READ: reading 0x00000036627fe2c0-0x00000036627fe320 96 byte(s) within 0x00000036627fe220-0x00000036627fe420
2020-12-07T02:55:07.8829450Z 4: # 0 ntdll.dll!RtlCaptureContext (0x00007ffe87809554 )
2020-12-07T02:55:07.8830075Z 4: ??:0
2020-12-07T02:55:07.8830696Z 4: # 1 ntdll.dll!RtlUnwindEx (0x00007ffe87795428 )
2020-12-07T02:55:07.8831255Z 4: ??:0
2020-12-07T02:55:07.8831916Z 4: # 2 ntdll.dll!RtlImageNtHeaderEx (0x00007ffe877949d3 )
2020-12-07T02:55:07.8832556Z 4: ??:0
2020-12-07T02:55:07.8833151Z 4: # 3 __longjmp_internal (0x00007ff6e1cd263a )
2020-12-07T02:55:07.8833940Z 4: d:\agent\_work\2\s\src\vctools\crt\vcruntime\src\eh\amd64\longjmp.asm:144
2020-12-07T02:55:07.8834749Z 4: # 4 __scrt_common_main_seh (0x00007ff6e1cc1918 )
2020-12-07T02:55:07.8835584Z 4: d:\agent\_work\2\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
2020-12-07T02:55:07.8836690Z 4: # 5 our_top_handler (0x00007ff6e1cc102a )
2020-12-07T02:55:07.8837624Z 4: d:\a\drmemory\drmemory\tests\malloc.c:75
2020-12-07T02:55:07.8838763Z 4: # 6 KERNELBASE.dll!UnhandledExceptionFilter (0x00007ffe83ca6227 )
2020-12-07T02:55:07.8839801Z 4: ??:0
2020-12-07T02:55:07.8840354Z 4: # 7 ntdll.dll!memset (0x00007ffe8780d963 )
2020-12-07T02:55:07.8840915Z 4: ??:0
2020-12-07T02:55:07.8841507Z 4: # 8 ntdll.dll!_C_specific_handler (0x00007ffe877f60d6 )
2020-12-07T02:55:07.8842277Z 4: ??:0
2020-12-07T02:55:07.8842913Z 4: # 9 ntdll.dll!RtlUserThreadStart (0x00007ffe877b1791 )
2020-12-07T02:55:07.8844247Z 4: ??:0
2020-12-07T02:55:07.8844797Z 4: Note: @0:00:02.369 in thread 4856
2020-12-07T02:55:07.8845354Z 4: Note: instruction: fxrstor 0x00000100(%rcx)
2020-12-07T02:55:07.8845811Z 4:
2020-12-07T02:55:07.8846907Z 4: Error #11: UNINITIALIZED READ: reading 0x00000036627fdf1a-0x00000036627fdf20 6 byte(s) within 0x00000036627fdf10-0x00000036627fdf38
2020-12-07T02:55:07.8848039Z 4: # 0 ntdll.dll!RtlCaptureContext (0x00007ffe8780966d )
2020-12-07T02:55:07.8849060Z 4: ??:0
2020-12-07T02:55:07.8849793Z 4: # 1 ntdll.dll!RtlUnwindEx (0x00007ffe87795428 )
2020-12-07T02:55:07.8850373Z 4: ??:0
2020-12-07T02:55:07.8851007Z 4: # 2 ntdll.dll!RtlImageNtHeaderEx (0x00007ffe877949d3 )
2020-12-07T02:55:07.8851665Z 4: ??:0
2020-12-07T02:55:07.8852236Z 4: # 3 __longjmp_internal (0x00007ff6e1cd263a )
2020-12-07T02:55:07.8853043Z 4: d:\agent\_work\2\s\src\vctools\crt\vcruntime\src\eh\amd64\longjmp.asm:144
2020-12-07T02:55:07.8853819Z 4: # 4 __scrt_common_main_seh (0x00007ff6e1cc1918 )
2020-12-07T02:55:07.8854632Z 4: d:\agent\_work\2\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
2020-12-07T02:55:07.8855404Z 4: # 5 our_top_handler (0x00007ff6e1cc102a )
2020-12-07T02:55:07.8856323Z 4: d:\a\drmemory\drmemory\tests\malloc.c:75
2020-12-07T02:55:07.8857375Z 4: # 6 KERNELBASE.dll!UnhandledExceptionFilter (0x00007ffe83ca6227 )
2020-12-07T02:55:07.8858279Z 4: ??:0
2020-12-07T02:55:07.8858889Z 4: # 7 ntdll.dll!memset (0x00007ffe8780d963 )
2020-12-07T02:55:07.8859783Z 4: ??:0
2020-12-07T02:55:07.8860424Z 4: # 8 ntdll.dll!_C_specific_handler (0x00007ffe877f60d6 )
2020-12-07T02:55:07.8861015Z 4: ??:0
2020-12-07T02:55:07.8861700Z 4: # 9 ntdll.dll!RtlUserThreadStart (0x00007ffe877b1791 )
2020-12-07T02:55:07.8862362Z 4: ??:0
2020-12-07T02:55:07.8862767Z 4: Note: @0:00:02.372 in thread 4856
2020-12-07T02:55:07.8863326Z 4: Note: instruction: iret %rsp (%rsp) -> %rsp
```

Showing up in many more tests as well.
```
$ grep -c '# 0 ntdll.dll!RtlCaptureContext' vs2017-64/5_Run\ Suite.txt
48
```