mesa: fix type for array indexing validation

mesa: fix type for array indexing validation

parse_program_resource_name returns -1 when the index is invalid this needs to
be tested before assigning the value to the unsigned array_index.

In link_varyings.cpp (the other place parse_program_resource_name is used) after
the -1 check is done the value is just assigned to an unsigned variable so it
seems long is just used so we can return the -1 rather than actually expecting
index values to be ridiculously large.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>