pxtp crashes when launched as a symlink

[Joseba Echevarria García]

When the user creates a symlink (in Linux) to pxtp with a name other than pxtp (say, tikibrawl), line 104 in src/main.c crashes because strstr() in line 102 failed (and we didn't check).

This is because in the strstr instruction appexefullpath contains the resolved path for the link (/home/joseba/cvs/pixtudio/projects/cmake/bin/gnulinux32/pxtp) whereas appexename contains the filename for the link itself (tikibrawl) and ptr is, therefore, NULL.