cmJSONState: Avoid UB on empty error message

The caller for `GetErrorMessage` _should_ only be calling this method
if there actual errors (i.e., `!errors.empty()`). However, in the
incidental absence of the intended usage, avoid `pop_back` on an empty
string.

Source/cmJSONState.cxx

@@ -78,7 +78,9 @@ std::string cmJSONState::GetErrorMessage(bool showContext)
       message = cmStrCat(message, GetJsonContext(loc), '\n');
     }
   }
-  message.pop_back();
+  if (!message.empty()) {
+    message.pop_back();
+  }
   return message;
 }