Don't use memchr() on strings potentially shorter than 4 bytes.

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>

(cherry picked from commit 7f48452d84)

src/ui/inputwin.c

@@ -180,11 +180,15 @@ _inp_slashguard_check(void)
         return false;
     if (!prefs_get_boolean(PREF_SLASH_GUARD))
         return false;
-    if (memchr(inp_line + 1, '/', 3)) {
-        cons_show("Your text contains a slash in the first 4 characters");
-        free(inp_line);
-        inp_line = NULL;
-        return true;
+    size_t n = 1;
+    while (inp_line[n] != '\0' && n < 4) {
+        if (inp_line[n] == '/') {
+            cons_show("Your text contains a slash in the first 4 characters");
+            free(inp_line);
+            inp_line = NULL;
+            return true;
+        }
+        n++;
     }
     return false;
 }