Merge pull request #2910 from cptjazz/compiler_warnings

Fix undefined behaviour in left-shift.

This PR fixes a left-shift that relied on undefined behaviour.

Clang emitted the following warning (prior to this patch):
```
sgen-descriptor.c:189:54: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
   return desc | VECTOR_SUBTYPE_REFS | ((gssize)(-1) << 16);
                                        ~~~~~~~~~~~~ ^
```