avformat/fitsenc: validate input pixel format

Fixes CID #1416961 and #1416962

libavformat/fitsenc.c

@@ -106,6 +106,8 @@ static int write_image_header(AVFormatContext *s)
             }
             bzero = 32768;
             break;
+        default:
+            return AVERROR(EINVAL);
     }
 
     if (fitsctx->first_image) {
@@ -166,7 +168,9 @@ static int write_image_header(AVFormatContext *s)
 
 static int fits_write_packet(AVFormatContext *s, AVPacket *pkt)
 {
-    write_image_header(s);
+    int ret = write_image_header(s);
+    if (ret < 0)
+        return ret;
     avio_write(s->pb, pkt->data, pkt->size);
     return 0;
 }