Protect pointer dereference to avoid a segfault when running headless.

When using libxbmc.so in a headless setup for library scans, dialog will be NULL. See:
http://forum.xbmc.org/showthread.php?tid=132919&pid=1320914#pid1320914
Merge pull request #2171 from popcornmix/headless_segfault

Protect pointer dereference to avoid a segfault when running headless.