avformat/nistspheredec: Clear buffer

Fixes: use-of-uninitialized-value
Fixes: 42537627/clusterfuzz-testcase-minimized-fuzzer_protocol_memory-6515855798632448-cut

Found-by: ossfuzz
Reported-by: Kacper Michajlow
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 898f6582eb)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/nistspheredec.c

@@ -35,7 +35,7 @@ static int nist_probe(const AVProbeData *p)
 
 static int nist_read_header(AVFormatContext *s)
 {
-    char buffer[256], coding[32] = "pcm", format[32] = "01";
+    char buffer[256]= {0}, coding[32] = "pcm", format[32] = "01";
     int bps = 0, be = 0;
     int32_t header_size = -1;
     AVStream *st;