avcodec/bitstream: Rewrite code to avoid triggering compiler warning

Clang infers from the existence of a default case that said case can be
taken. In case of libavcodec/bitstream.c said default case consisted of
an av_assert1 that evaluates to nothing in case of the ordinary assert
level. In this case (that doesn't happen) a variable wouldn't be
initialized, so Clang emitted Wsometimes-uninitialized warnings.
Solve this by making sure that the default path also initializes
the aforementioned variable.
Reviewed-by: NPaul B Mahol <onemda@gmail.com>
Signed-off-by: NAndreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/bitstream.c

@@ -104,10 +104,10 @@ void ff_copy_bits(PutBitContext *pb, const uint8_t *src, int length)
         v = *(const uint16_t *)ptr;                         \
         break;                                              \
     case 4:                                                 \
+    default:                                                \
+        av_assert1(size == 4);                              \
         v = *(const uint32_t *)ptr;                         \
         break;                                              \
-    default:                                                \
-        av_assert1(0);                                      \
     }                                                       \
 }