j2k & jpeg2000dec: reset numX/Ytiles if tiles failed allocation

Fixes null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/j2kdec.c

@@ -957,6 +957,8 @@ static int decode_codestream(Jpeg2000DecoderContext *s)
         switch (marker){
         case JPEG2000_SIZ:
             ret = get_siz(s);
+            if (!s->tile)
+                s->numXtiles = s->numYtiles = 0;
             break;
         case JPEG2000_COC:
             ret = get_coc(s, codsty, properties);

libavcodec/jpeg2000dec.c

@@ -1181,6 +1181,8 @@ static int jpeg2000_read_main_headers(Jpeg2000DecoderContext *s)
         switch (marker) {
         case JPEG2000_SIZ:
             ret = get_siz(s);
+            if (!s->tile)
+                s->numXtiles = s->numYtiles = 0;
             break;
         case JPEG2000_COC:
             ret = get_coc(s, codsty, properties);