geyes: fix crash/hang when theme-path is empty and "config" is a directory

[u9g]

The Eyes applet crashes (or hangs at 100% CPU) at panel start-up when its theme-path key is unset and the process's working directory contains an entry named config. Seen on Ubuntu 22.04 with mate-applets 1.26.0; the affected code is unchanged on master.

Cause

Two things combine:

1. properties_load() checks the theme path for NULL, but g_settings_get_string() returns "" for the unset key. g_build_filename("", "config") then yields the relative path config, resolved against the working directory. Under D-Bus activation that is the user's home directory.
2. fopen() happily opens a directory. Reads then fail with EISDIR, which never sets the EOF flag, so the feof()-driven loop in parse_theme_file() runs on an uninitialised buffer. strtok() returns NULL and strncmp() dereferences it.

Backtrace from the crash report (frame 1-3 are in the applet):

#0  __strcmp_avx2 ()                     rdi=0x0, rsi="wall-thickness"
#1  parse_theme_file / load_theme
#4  mate_panel_applet_marshal_BOOLEAN__STRING ()

Fix

• Treat an empty theme-path like NULL and fall back to Default-tiny.
• Require the theme's config to be a regular file before opening it.
• Drive the parser loop with fgets(), skip lines where strtok() returns NULL, and read values with a second strtok() call instead of walking past the NUL that the first call wrote over =.

Testing

• All seven shipped themes parse to the same wall-thickness, num-eyes and pixmap values as before the change.
• With a directory named config in the working directory and an empty theme-path, the old build hangs or segfaults; the patched build loads Default-tiny and draws the eyes on the panel.