When running profanity under Valgrind with slashguard enabled, the
following error could occur:
```
[...]
==4021347== Invalid read of size 1
==4021347== at 0x4851F49: memchr (vg_replace_strmem.c:986)
==4021347== by 0x45CEAD: _inp_slashguard_check (inputwin.c:183)
==4021347== by 0x45CEAD: inp_readline (inputwin.c:225)
==4021347== by 0x431184: prof_run (profanity.c:121)
==4021347== by 0x42C609: main (main.c:176)
==4021347== Address 0xe850883 is 0 bytes after a block of size 3 alloc'd
==4021347== at 0x48477C4: malloc (vg_replace_malloc.c:446)
[...]
```
`memchr()` requires the complete memory that shall be searched to be
accessible. Using `strchr()` could work for shorter strings, but we only
want to search in the first 4 chars.
Instead of somehow working around those limitations, simply search manually
in the first 4 bytes.
Fixes: 3c56b289 ("Add slashguard feature")
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>
7f48452d84