Fix memory leak in edit-font toggle style

FONT_OT_style_toggle returned `true` instead of the canceled flag
causing it to run as a modal operator which leak memory from it's
reports when called from Python.

source/blender/editors/curve/editfont.cc

@@ -1003,7 +1003,7 @@ static int toggle_style_exec(bContext *C, wmOperator *op)
     clear = (cu->curinfo.flag & style) == 0;
     return set_style(C, style, clear);
   }
-  return true;
+  return OPERATOR_CANCELLED;
 }
 
 void FONT_OT_style_toggle(wmOperatorType *ot)