glcpp: Fix undefined behaviour in glcpp
Specifically, fix this error (which is covered in existing tests):
../src/compiler/glsl/glcpp/pp.c:198:28: runtime error: applying non-zero offset 1 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/compiler/glsl/glcpp/pp.c:198:28 in
Closes: #4477 (closed)