aacdec: Fix check of return value of NeAACDecInit2().

aacdec: Fix check of return value of NeAACDecInit2().

This function returns (char)-1 on errors and zero on success. Checking
whether the return value is negative is incorrect on systems where
char is unsigned.  On raspberry/ARM, gcc complains:

	aacdec_filter.c:126:6: warning: comparison is always false due to limited range of data type [-Wtype-limits]

Fix this by checking agains zero instead.