avcodec/microdvddec: Check for string end in 'P' case

Fixes out of array read
Fixes: a9502b60f4cecc19475382aee255f73c/asan_heap-oob_1e87fba_2548_a8ad47f6dde36644fe9cdc444d4632d0.sub

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit c719cd6cf7)

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/microdvddec.c

@@ -164,6 +164,8 @@ static char *microdvd_load_tags(struct microdvd_tag *tags, char *s)
 
         /* Position */
         case 'P':
+            if (!*s)
+                break;
             tag.persistent = MICRODVD_PERSISTENT_ON;
             tag.data1 = (*s++ == '1');
             if (*s != '}')