mesa: check that uniform exists in glUniform* functions

mesa: check that uniform exists in glUniform* functions

Remap table for uniforms may contain empty entries when using explicit
uniform locations. If no active/inactive variable exists with given
location, remap table contains NULL.

v2: move remap table bounds check before existence check (Ian Romanick)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Tested-by: Erik Faye-Lund <kusmabite@gmail.com> (v1)
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83574