put_bits: fix invalid shift by 32 in flush_put_bits()

If flush_put_bits() is called when the 32-bit buffer is empty,
e.g. after writing a multiple of 32 bits, and invalid shift by
32 is performed.  Since flush_put_bits() is called infrequently,
this additional check should have negligible performance impact.

Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit ac6eab1496)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/put_bits.h

@@ -100,7 +100,8 @@ static inline void flush_put_bits(PutBitContext *s)
     align_put_bits(s);
 #else
 #ifndef BITSTREAM_WRITER_LE
-    s->bit_buf<<= s->bit_left;
+    if (s->bit_left < 32)
+        s->bit_buf<<= s->bit_left;
 #endif
     while (s->bit_left < 32) {
         /* XXX: should test end of buffer */