get_xbits: assert validity of the number of bits

similar is already done in the other get_bits() functions
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/get_bits.h

@@ -214,6 +214,7 @@ static inline int get_xbits(GetBitContext *s, int n)
     register int sign;
     register int32_t cache;
     OPEN_READER(re, s);
+    av_assert2(n>0 && n<=25);
     UPDATE_CACHE(re, s);
     cache = GET_CACHE(re, s);
     sign  = ~cache >> 31;