StringUtils: Check for NULL-pointer in EqualsNoCase()

Stumbled upon this while testing with master after the CStdString drop:

Looks like that in https://github.com/xbmc/xbmc/commit/abba6f5dc5e95325db9d88ddfc5da4c55a4d53ba#diff-cd0ebb1146353f99c6dd421a4f733034R621 "xhResults.Element()->Attribute("sorted")" can result in a NULL return. This caused a segfault in StringUtils::EqualsNoCase() as that case is not checked there. This little patch checks that.

Not sure if NULL == NULL should be true or if any of the pointer being NULL should resolve to false, so just did it like this ;-)