glsl/linker: fix output variable overlap check

glsl/linker: fix output variable overlap check

Prevent an overflow caused by too many output variables. To limit the
scope of the issue, write to the assigned array only for the non-ES
fragment shader path, which is the only place where it's needed.

Since the function will bail with an error when output variables with
overlapping components are found, (max # of FS outputs) * 4 is an upper
limit to the space we need.

Found by address sanitizer.

Fixes dEQP-GLES3.functional.attribute_location.bind_aliasing.*

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>