avcodec/bitstream_filters: check the input argument of av_bsf_get_by_name() for NULL

Fixes crashes like "ffmpeg -h bsf" caused by passing NULL to strcmp()
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 3258cc65)

libavcodec/bitstream_filters.c

@@ -58,6 +58,9 @@ const AVBitStreamFilter *av_bsf_get_by_name(const char *name)
 {
     int i;
 
+    if (!name)
+        return NULL;
+
     for (i = 0; bitstream_filters[i]; i++) {
         const AVBitStreamFilter *f = bitstream_filters[i];
         if (!strcmp(f->name, name))