i965: Silence warning.

i965: Silence warning.

intel_asm_annotation.c: In function ‘annotation_insert_error’:
intel_asm_annotation.c:214:18:
warning: ‘ann’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
       ann->error = ralloc_strdup(annotation->mem_ctx, error);
                         ^

I initially tried changing the type of ann_count to unsigned (is
currently int), since that in addition to the check that it's non-zero
at the beginning of the function seems sufficient to prove that it must
be greater than zero. Unfortunately that wasn't sufficient.