Fix undefined behaviour in CommandBufferGL.cpp

[hexagonrecursion]

By compiling with sanitisers enabled I have detected a number of places where we run into undefined behaviour. The patch for the first is in #5692 (thanks to Web-eWorks). This is a patch for the second issue I detected.

1. mat->m_pushConstants.get() is sometimes null.
2. I set a breakpoint in a debugger and it turned out that s->GetConstantStorageSize() == 0
3. The behaviour of memcpy(dst, src, count) is undefined if either of the pointers is null even if count == zero

Fix: add a null check.

Here is how I compile with sanitisers.
1 Make sure build/ does not exist (because cmake does not update CXXFLAGS for build directories that have already been populated). Alternatively: build in build-with-sanitizers/
2. CXXFLAGS="-fsanitize=address,pointer-compare,pointer-subtract,undefined" LDFLAGS="$CXXFLAGS" cmake -S . -B build/ && make -j2 -C build/

[Web-eWorks]

Apologies for the delay on reviewing these PRs - thanks for digging into it! I'm always glad to see someone take the time to validate the behavior of the compiled code.

[impaktor]

@hexagonrecursion Thanks a bunch for the work you've done! Please let us know if you want to be in AUTHORS.txt, and what to put.