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.

(cherry picked from commit 2d9df5a)

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);
 }