Silence gcc warning

This complained that the variable might be uninitialized *right* after
the check that it wasn't, because it doesn't understand maybe_t.

src/highlight.cpp

@@ -766,8 +766,16 @@ static void color_string_internal(const wcstring &buffstr, highlight_spec_t base
     }
 
     // Error on unclosed quotes.
-    if (unclosed_quote_offset) {
+    if (unclosed_quote_offset.has_value()) {
+        // gcc complains this is uninitialized
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
         colors[*unclosed_quote_offset] = highlight_role_t::error;
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
     }
 }