goption: Relax assertion to avoid being broken by kdeinit5

kdeinit5 overwrites argv, which in turn results in /proc/self/cmdline
being overwritten. It seems that this is done in a way that does not
necessarily guarantee that /proc/self/cmdline will end up NUL-terminated.

However, g_file_get_contents() is documented to fill a buffer of size
len + 1, where buffer[len] == '\0', even if the file's actual contents
(from buffer[0] to buffer[len-1] inclusive) did not include a NUL;
so we can safely relax this assertion slightly.

Resolves: https://gitlab.gnome.org/GNOME/glib/issues/1923
Signed-off-by: Simon McVittie <smcv@collabora.com>

glib/goption.c

@@ -1833,8 +1833,10 @@ platform_get_argv0 (void)
 			    NULL))
     return NULL;
 
-  /* Sanity check for a NUL terminator. */
-  g_assert (memchr (cmdline, 0, len));
+  /* g_file_get_contents() guarantees to put a NUL immediately after the
+   * file's contents (at cmdline[len] here), even if the file itself was
+   * not NUL-terminated. */
+  g_assert (memchr (cmdline, 0, len + 1));
 
   /* We could just return cmdline, but I think it's better
    * to hold on to a smaller malloc block; the arguments