glsl: Return NULL on _slan_gen_asm error path on non-debug builds.

Exit the function early on the error path, instead of continuing, to
prevent a null pointer dereference later on.

src/mesa/shader/slang/slang_codegen.c

@@ -1894,6 +1894,7 @@ _slang_gen_asm(slang_assemble_ctx *A, slang_operation *oper,
       _mesa_problem(NULL, "undefined __asm function %s\n",
                     (char *) oper->a_id);
       assert(info);
+      return NULL;
    }
    assert(info->NumParams <= 3);