Fix compile error when compiling with whole program optimization enabled

[jan-kelemen]

Compiling with /GL (Whole program optimization) and /LTCG (Link-time code generation) flags on MSVC 2022 causes an error/warning (depends if /sdl flag is also used) to be issued:

C:\Users\puma\.conan2\p\b\spirv2755296b2b5cb\b\src\source\table.cpp(60): error C4703: potentially uninitialized local pointer variable 'ext_inst_table' used [C:\Users\puma\.conan2\p\b\spirv2755296b2b5cb\b\bui
ld\source\SPIRV-Tools-shared.vcxproj]
LINK : fatal error LNK1257: code generation failed [C:\Users\puma\.conan2\p\b\spirv2755296b2b5cb\b\build\source\SPIRV-Tools-shared.vcxproj]

The code should actually be OK, as spvExtInstTableGet does check the same set of values for the env variable as is in the switch case above the modified code which does an early return.