msOGRShapeFromWKT(): fix memleak in error code path

Likely fix for https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=52223
(couldn't reproduce the leak with the reproducer and current main, but
this commit does fix a leak)

mapogr.cpp

@@ -5485,7 +5485,7 @@ shapeObj *msOGRShapeFromWKT(const char *string)
                             wkbFlatten(OGR_G_GetGeometryType(hGeom)) )
       == MS_FAILURE ) {
     free( shape );
-    return NULL;
+    shape = NULL;
   }
 
   OGR_G_DestroyGeometry( hGeom );