glsl: return cloned signature, not the builtin one

The builtin data can get released with a glReleaseShaderCompiler call.
We're careful everywhere to clone everything that comes out of builtins
except here, where we accidentally return the signature belonging to the
builtin version, rather than the locally-cloned one.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Tested-by: Rob Herring <robh@kernel.org>
Cc: mesa-stable@lists.freedesktop.org

src/compiler/glsl/ast_function.cpp

@@ -560,7 +560,8 @@ done:
 	    state->symbols->add_global_function(f);
 	    emit_function(state, f);
 	 }
-	 f->add_signature(sig->clone_prototype(f, NULL));
+	 sig = sig->clone_prototype(f, NULL);
+	 f->add_signature(sig);
       }
    }
    return sig;