Work around gcc9 warning bug

[Zack Weinberg]

On 30 Apr 2019, Alan Modra wrote:
> * wrstabs.c (stab_start_class_type): Add assert to work around gcc9 warning.  Tidy.

Could you please backport this patch to the binutils 2.32 branch at
least?  The gcc9 warning is breaking glibc's "build-many-glibcs"
automation for exhaustive cross-compilation testing.  As part of
bootstrapping cross compilation environments, this builds a native gcc
9 and then tries to use it to build cross-binutils, currently from the
2.32 branch.

zw