glsl: assert base_alignment > 0 for records

From GLSL 1.50 spec, section 4.1.8 "Structures":

"Structures must have at least one member declaration."

So the base_alignment should be higher than zero.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/glsl/glsl_types.cpp

@@ -1511,6 +1511,7 @@ glsl_type::std430_base_alignment(bool row_major) const
          base_alignment = MAX2(base_alignment,
                                field_type->std430_base_alignment(field_row_major));
       }
+      assert(base_alignment > 0);
       return base_alignment;
    }
    assert(!"not reached");