avcodec/movtextenc: Check for existence of font name before using it

Fixes crashes if the font name is NULL (which it is if a \fn tag
is not followed by a font name).

Signed-off-by: Charlie Monroe <charlie@charliemonroe.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/movtextenc.c

@@ -494,8 +494,10 @@ static void mov_text_alpha_cb(void *priv, int alpha, int alpha_id)
 
 static uint16_t find_font_id(MovTextContext *s, const char *name)
 {
-    int i;
-    for (i = 0; i < s->font_count; i++) {
+    if (!name)
+        return 1;
+
+    for (int i = 0; i < s->font_count; i++) {
         if (!strcmp(name, s->fonts[i]))
             return i + 1;
     }