lua: fix memory leak in set method

scan-build from clang version 9 has reported following issue:

 uci.c:624:12: warning: Potential leak of memory pointed to by 's'
       return luaL_error(L, "Cannot set an uci option to an empty table value");
              ^~~~~~~~~~

valgrind confirmed it on the supplied test case:

 ==31013== 8 bytes in 1 blocks are definitely lost in loss record 1 of 1
 ==31013==    by 0x56C49B9: strdup (strdup.c:42)

Signed-off-by: Petr Štetiar <ynezz@true.cz>

1 files changed, 3 insertions, 1 deletions

diff --git a/lua/uci.c b/lua/uci.c
index 42fe4b7..ad08299 100644
--- a/lua/uci.c
+++ b/lua/uci.c
@@ -620,8 +620,10 @@ uci_lua_set(lua_State *L)
 	case 4:
 		/* Format: uci.set("p", "s", "o", "v") */
 		if (lua_istable(L, nargs)) {
-			if (lua_rawlen(L, nargs) < 1)
+			if (lua_rawlen(L, nargs) < 1) {
+				free(s);
 				return luaL_error(L, "Cannot set an uci option to an empty table value");
+			}
 			lua_rawgeti(L, nargs, 1);
 			ptr.value = luaL_checkstring(L, -1);
 			lua_pop(L, 1);