Read after free in TypeSpec::struct_list()

### Problem
I compile a shader with a struct using `OSLCompiler`. When `OSLCompiler` is destructed it performs `SymbolTable::delete_syms()` which clears `TypeSpec::struct_list()`.

Some operations, like `ShadingSystem::oslquery()` and `Symbol::print()` access entries in `TypeSpec::struct_list()` which was deleted when `OSLCompiler` went out of scope.

Related to https://github.com/AcademySoftwareFoundation/OpenShadingLanguage/issues/1427

### Steps to Reproduce
The problem can be triggered with testshade, Visual Studio 2019, debug configuration.

1. Run testshade
```
>bin\debug\testshade --inbuffer ..\osl-v1.12.6.0-beta\testsuite\struct\test.osl
```
2. Result
```
0# boost::stacktrace::basic_stacktrace >::init at C:\WORK\GIT\strala\winbuild\install\deps-debug\include\boost-1_78\boost\stacktrace\stacktrace.hpp:76
1# boost::stacktrace::basic_stacktrace >::basic_stacktrace > at C:\WORK\GIT\strala\winbuild\install\deps-debug\include\boost-1_78\boost\stacktrace\stacktrace.hpp:129
2# OpenImageIO_v2_3::Sysutil::stacktrace at C:\WORK\GIT\strala\winbuild\src\oiio-v2.3.12.0\src\libutil\sysutil.cpp:660
3# OpenImageIO_v2_3::stacktrace_signal_handler at C:\WORK\GIT\strala\winbuild\src\oiio-v2.3.12.0\src\libutil\sysutil.cpp:680
4# raise in ucrtbased
5# setmbcp in ucrtbased
6# CrtSetReportHookW2 in ucrtbased
7# VCrtDbgReportA in ucrtbased
8# CrtDbgReport in ucrtbased
9# std::vector,std::allocator > >::operator[] at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\vector:1552
10# OSL_v1_12::pvt::TypeSpec::structspec at C:\WORK\GIT\osl-v1.12.6.0-beta\src\include\osl_pvt.h:175
11# OSL_v1_12::pvt::TypeSpec::structspec at C:\WORK\GIT\osl-v1.12.6.0-beta\src\include\osl_pvt.h:169
12# OSL_v1_12::ShadingSystem::oslquery at C:\WORK\GIT\osl-v1.12.6.0-beta\src\liboslexec\shadingsys.cpp:4216
13# setup_output_images at C:\WORK\GIT\osl-v1.12.6.0-beta\src\testshade\testshade.cpp:1030
14# test_shade at C:\WORK\GIT\osl-v1.12.6.0-beta\src\testshade\testshade.cpp:2043
15# main at C:\WORK\GIT\osl-v1.12.6.0-beta\src\testshade\testshademain.cpp:29
16# invoke_main at D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:79
17# __scrt_common_main_seh at D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
18# __scrt_common_main at D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:331
19# mainCRTStartup at D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_main.cpp:17
20# BaseThreadInitThunk in KERNEL32
21# RtlUserThreadStart in ntdll
```

### Versions

* OSL branch/version: v1.12.6.0-beta
* OS: Windows 11
* C++ compiler: Visual Studio 2019 16.11.18
* LLVM version: 13.0.1
* OIIO version: v1.11.17.0

Run the provided testshade command in the stated Windows debug configuration, then inspect TypeSpec::structspec in src/include/osl_pvt.h and its use from src/liboslexec/shadingsys.cpp. Trace the OSLCompiler destruction path through SymbolTable::delete_syms(); done means the reproduced read-after-free no longer occurs when oslquery() or Symbol::print() accesses struct_list().

Written by the indexing model from the issue text.