Ensure the return value from get_server_path is freeable

[frankleonrose]

While testing in support of #2252 I ran into this crash bug which looks like this:

% x/app/scrcpy
2021-04-22 12:50:31.596 scrcpy[82192:39073792] INFO: scrcpy 1.16 <https://github.com/Genymobile/scrcpy>
2021-04-22 12:50:31.596 scrcpy[82192:39073792] ERROR: Could not get executable path, using scrcpy-server from current directory
scrcpy(82192,0x11122fdc0) malloc: *** error for object 0x10a439c2c: pointer being freed was not allocated
scrcpy(82192,0x11122fdc0) malloc: *** set a breakpoint in malloc_error_break to debug
scrcpy-server: 1 file pushed. 4.5 MB/s (33622 bytes in 0.007s)
zsh: abort      x/app/scrcpy --window-borderless

The fix is simply that in the case where get_server_path returns just SERVER_FILENAME, it must be duplicated, because the caller expects to be able to free the returned value. If the duplication fails, we return NULL which indicates failure, so no further error handling is necessary, unless we wanted to log something about that extremely rare situation.

[rom1v]

Yes, thank you 👍 LGTM.

However, I'm surprised you execute this code. Did you explicitly enabled -Dportable=true in your meson command?

[rom1v]

I rebased on dev (so I also replaced SDL_strdup() by strdup(), because of changes on that branch, and merged: aaf7875

[frankleonrose]

Yes, I did specify portable because I was testing the 1.16 version and had the compatible server local.

Thanks for taking the fix. Sorry I hadn't targeted dev.

[rom1v]

I did specify portable because I was testing the 1.16 version and had the compatible server local.

OK. Just for info, instead, you could just use -Dprebuilt_server=... in your meson command, or set SCRCPY_SERVER_PATH at runtime.