avcodec/get_bits: add assertion to limit ouptut value of get_bits

Should fix the following Coverity false positives:

Coverity CID #1415651.
Coverity CID #1420392.
Coverity CID #1420473.
Coverity CID #1433770.
Coverity CID #1435320.
Coverity CID #1439573.
Coverity CID #1439580.
Coverity CID #1439588.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavcodec/get_bits.h

@@ -403,6 +403,7 @@ static inline unsigned int get_bits(GetBitContext *s, int n)
     LAST_SKIP_BITS(re, s, n);
     CLOSE_READER(re, s);
 #endif
+    av_assert2(tmp < UINT64_C(1) << n);
     return tmp;
 }