glsl/nir: Avoid overflow when setting max_uniform_location

Don't use the UNMAPPED_UNIFORM_LOC (-1) to set the unsigned
max_uniform_location.  Those unmapped uniforms don't have to be
accounted at this point.

Fixes: 7a9e5cdf ("nir/linker: Add gl_nir_link_uniforms()")
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
(cherry picked from commit 4f33f96c)