st/glsl_to_nir: fix incorrect arrary access

This fixes a segfault when we try to access the array using a
-1 when the array wasn't allocated in the first place.

Before 7536af670b we would just access a pre-allocated array
that was also load/stored to/from the shader cache. But now the
cache will no longer allocate these arrays if they are empty.
The change resulted in tests such as the following segfaulting
when run with a warm shader cache.

tests/spec/arb_arrays_of_arrays/execution/sampler/fs-struct-const-index.shader_test

src/mesa/state_tracker/st_glsl_to_nir.cpp

@@ -288,13 +288,16 @@ st_nir_assign_uniform_locations(struct gl_context *ctx,
       } else {
          loc = st_nir_lookup_parameter_index(prog->Parameters, uniform->name);
 
-         if (ctx->Const.PackedDriverUniformStorage) {
+         /* We need to check that loc is not -1 here before accessing the
+          * array. It can be negative for example when we have a struct that
+          * only contains opaque types.
+          */
+         if (loc >= 0 && ctx->Const.PackedDriverUniformStorage) {
             loc = prog->Parameters->ParameterValueOffset[loc];
          }
       }
 
       uniform->data.driver_location = loc;
-
       max = MAX2(max, loc + type_size(uniform->type));
    }
    *size = max;