fix i3bar crashing when I3SOCK present (#2471)

When I3SOCK is present, socket_path might be a pointer to an
environment variable, which cannot be free'd in line 157. This
commit duplicates the string if I3SOCK is present, thus making
socket_path a free-able pointer again.

i3bar/src/main.c

@@ -93,6 +93,9 @@ int main(int argc, char **argv) {
     int opt;
     int option_index = 0;
     char *socket_path = getenv("I3SOCK");
+    if (socket_path != NULL) {
+        socket_path = sstrdup(socket_path);
+    }
     char *i3_default_sock_path = "/tmp/i3-ipc.sock";
 
     /* Initialize the standard config to use 0 as default */