nir: avoid segfault when ssa src not found
nir: avoid segfault when ssa src not found

Without this the following line will segfault and we don't get to
see the results of the validate_assert() above.

Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler