i3-input: fix (irrelevant) memory leak

Given that the code was exit(0)ing directly after using that memory,
it’s not like this has any effect. However, less false positives on the
clang-analyze report pages is a good thing.

i3-input/main.c

@@ -214,6 +214,8 @@ static void finish_input() {
 
     ipc_send_message(sockfd, strlen(full), 0, (uint8_t*)full);
 
+    free(full);
+
 #if 0
     free(command);
     return 1;