flow/var: Release key storage

Issue: 7466

This commit releases the memory for the flow variable "key" when
the flow variable is of type string. The key is allocated in the Lua
extension logic.

src/flow-var.c

@@ -166,6 +166,7 @@ void FlowVarFree(FlowVar *fv)
     if (fv->datatype == FLOWVAR_TYPE_STR) {
         if (fv->data.fv_str.value != NULL)
             SCFree(fv->data.fv_str.value);
+        SCFree(fv->key);
     }
     SCFree(fv);
 }