fix assertion error when loading svg symbol from labelcache

fixes #4247
@woodbri please confirm, as the SVG loading code has changed since you
submitted this patch

mapsymbol.c

@@ -652,6 +652,15 @@ int msGetMarkerSize(symbolSetObj *symbolset, styleObj *style, double *width, dou
       if (MS_SUCCESS != msPreloadImageSymbol(MS_MAP_RENDERER(symbolset->map), symbol))
          return MS_FAILURE;
   }
+  if(symbol->type == MS_SYMBOL_SVG && !symbol->renderer_cache) {
+#ifdef USE_SVG_CAIRO
+     if(MS_SUCCESS != msPreloadSVGSymbol(symbol))
+        return MS_FAILURE;
+#else
+     msSetError(MS_SYMERR, "SVG symbol support is not enabled.", "msGetMarkerSize()");
+     return MS_FAILURE;
+#endif
+  }
   if(style->size == -1) {
       size = ( msSymbolGetDefaultSize(symbol) * scalefactor );
   }