SetFnt will throw an exception if the font parser doesn't return something valid.
NSAssert replaced so any environment can handle the exception.
The problem is a bit further down the setter: createFontChars can crash with a SIGSEGV when the config is not valid.
Merge pull request #669 from NickyWeber/bugfix/ccblabelbmfont_setfnt_crash

SetFnt will throw an exception if the font parser doesn't return something valid.