glsl2: Don't try to assign locations for samplers during linking.

Mesa will do the mapping at _mesa_add_sampler() time.  Fixes assertion
failures in debug builds, which might have caught real problems with
multiple samplers linked in a row.

src/glsl/linker.cpp

@@ -793,6 +793,9 @@ assign_uniform_locations(struct gl_shader_program *prog)
 	 if ((var == NULL) || (var->mode != ir_var_uniform))
 	    continue;
 
+	 if (var->type->is_sampler())
+	    continue;
+
 	 const unsigned vec4_slots = (var->component_slots() + 3) / 4;
 	 assert(vec4_slots != 0);