swr: validate backend state numAttributes
swr: validate backend state numAttributes

General protection and prevents us from smashing the stack
on the first clear state validation (a7b8d50bcb).  Fixes crash
using icc.

Reviewed-by: Bruce Cherniak <a5345730962d952f474b19e983398300e1cb5f29@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler