jpeg2000: Zero prec->cblk

Prevent use of uninitialized memory / valgrind failure

Found-by: ubitux
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/jpeg2000.c

@@ -424,9 +424,9 @@ int ff_jpeg2000_init_component(Jpeg2000Component *comp,
                 if (!prec->zerobits)
                     return AVERROR(ENOMEM);
 
-                prec->cblk = av_malloc_array(prec->nb_codeblocks_width *
-                                             (uint64_t)prec->nb_codeblocks_height,
-                                             sizeof(*prec->cblk));
+                prec->cblk = av_mallocz_array(prec->nb_codeblocks_width *
+                                              (uint64_t)prec->nb_codeblocks_height,
+                                              sizeof(*prec->cblk));
                 if (!prec->cblk)
                     return AVERROR(ENOMEM);
                 for (cblkno = 0; cblkno < prec->nb_codeblocks_width * prec->nb_codeblocks_height; cblkno++) {