glsl/linker: report linker errors for invalid explicit locations on inputs

glsl/linker: report linker errors for invalid explicit locations on inputs

We were assuming that if an input has an invalid explicit location it would
fail to link because it would not find the corresponding output, however,
since we look for the matching output by indexing the explicit_locations
array with the input location, we still need to ensure that we don't index
out of bounds.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>