libavformat/riffec: Zero-initialize channels in ff_get_wav_header

Clang's static analyzer complains that leaving the  variable
uninitialized could lead to a code path where the uninitialized value is
written to  at the end of this function.
This patch simply zero-initializes that variable to avoid that.
Signed-off-by: Will Cassella <cassew@google.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/riffdec.c

@@ -94,7 +94,7 @@ static void parse_waveformatex(AVFormatContext *s, AVIOContext *pb, AVCodecParam
 int ff_get_wav_header(AVFormatContext *s, AVIOContext *pb,
                       AVCodecParameters *par, int size, int big_endian)
 {
-    int id, channels;
+    int id, channels = 0;
     uint64_t bitrate = 0;
 
     if (size < 14) {