rts: free error message before returning

Fixes a memory leak in rts/linker/PEi386.c

(cherry picked from commit dd530bb7)

rts/linker/PEi386.c

@@ -1862,6 +1862,7 @@ ocGetNames_PEi386 ( ObjectCode* oc )
           if (result != NULL || dllInstance == 0) {
               errorBelch("Could not load `%s'. Reason: %s\n",
                          (char*)dllName, result);
+              stgFree((void*)result);
               return false;
           }