Fix a possible buffer overflow via user input from fgets

[primeos]

fgets reads at most n-1 characters but may need to store up to n characters due to the final null-character.

The crash could e.g. be triggered with this command: $ printf "000000000000000000000000000000000001" > "$SWAYSOCK.wob"

Which resulted in a buffer overflow: $ tail -f "$SWAYSOCK.wob" | ./wob buffer overflow detected : ./wob terminated Aborted (core dumped)

---

I noticed this after compiling wob, which gave me the following warning:

[11/12] Compiling C object 'wob@exe/wob.c.o'.'.ble_v1_lib.a.generated_.._wlr-layer-shell-unstable-v1.c.o'.
In file included from /nix/store/zg3y0jq36pd8xf8rd8rhj8bcpinyrs3s-glibc-2.27-dev/include/stdio.h:862,
                 from ../wob.c:21:
In function 'fgets',
    inlined from 'main' at ../wob.c:450:16:
/nix/store/zg3y0jq36pd8xf8rd8rhj8bcpinyrs3s-glibc-2.27-dev/include/bits/stdio2.h:260:9: warning: call to '__fgets_chk_warn' declared with attribute warning: fgets called with bigger size than length of destination buffer
  return __fgets_chk_warn (__s, __bos (__s), __n, __stream);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[12/12] Linking target wob.

With this fix wob prints "Received invalid input" and exits (return code 1) without a buffer overflow.

[francma]

Thanks :slightly_smiling_face: