Fix handling of invalid x-special/mate-icon-list drops

`p` actually could never be `0` (because of the NULL check on the
`memchr()` call), but the intended behavior is `*p == '\0'`: the
containing condition checks for either a truncated data (`*p == '\0'`)
or no geometry information (`*p == '\n'`).

I replaced the check to be `*p != '\n'` instead of `*p == '\0'` to
make this more robust as the actual issue is anything but a newline,
the fact it can only be a NUL otherwise is incidental to the enclosing
check, but not really relevant at this level.  This is also in line
with the actual error message.

Found by cppcheck: https://caja.mate-desktop.dev/2022-11-23-174623-5790-cppcheck@ae663c369cf2_desktop-no-overflow/16.html#line-204

0 files changed, 0 insertions, 0 deletions