loadGrid(): fix memleak on LABELFORMAT DD

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=52670

Author: rouault

mapfile.c

@@ -1083,8 +1083,10 @@ static int loadGrid( layerObj *pLayer )
         break; /* for string loads */
       case( LABELFORMAT ):
         if(getString(&(pLayer->grid->labelformat)) == MS_FAILURE) {
-          if(strcasecmp(msyystring_buffer, "DD") == 0) /* DD triggers a symbol to be returned instead of a string so check for this special case */
+          if(strcasecmp(msyystring_buffer, "DD") == 0) /* DD triggers a symbol to be returned instead of a string so check for this special case */ {
+            msFree(pLayer->grid->labelformat);
             pLayer->grid->labelformat = msStrdup("DD");
+          }
           else
             return(-1);
         }