avformat/aaxdec: Check string before strcmp()

Fixes: NULL ptr dereference
Fixes: 26508/clusterfuzz-testcase-minimized-ffmpeg_dem_AAX_fuzzer-5694725249826816

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/aaxdec.c

@@ -232,7 +232,7 @@ static int aax_read_header(AVFormatContext *s)
         int64_t col_offset;
         int flag, type;
 
-        if (strcmp(a->xcolumns[c].name, "data"))
+        if (!a->xcolumns[c].name || strcmp(a->xcolumns[c].name, "data"))
             continue;
 
         type = a->xcolumns[c].type;