Ensure FlatWGS attribute is not null before inspecting

[acowley]

The lack of this check was causing segfaults for me when compiling some things with hcc but not others. My bug hunt narrowed down to failing miopen-hip tests on my machine. The larger context is that I have all ROCm pieces compiled from source on NixOS, starting with a build environment including gcc-7.3.0.

Earlier logic in this function as found in upstream clang looked like this,

unsigned Min = FlatWGS ? FlatWGS->getMin() : 0;
unsigned Max = FlatWGS ? FlatWGS->getMax() : 0;

but the check that FlatWGS is not null was lost along the way.