jp2: convert: fix null pointer dereference

Tile components in a JP2 image might have null data pointer by defining a
zero component size (for example using large horizontal or vertical
sampling periods). This null data pointer leads to null image component
data pointer, causing crash when dereferenced without != null check in
imagetopnm.

Add != null check.

This commit addresses uclouvain#1152 (CVE-2018-18088).

src/bin/jp2/convert.c

@@ -2233,6 +2233,11 @@ int imagetopnm(opj_image_t * image, const char *outfile, int force_split)
                 opj_version(), wr, hr, max);
 
         red = image->comps[compno].data;
+        if (!red) {
+            fclose(fdest);
+            continue;
+        }
+
         adjustR =
             (image->comps[compno].sgnd ? 1 << (image->comps[compno].prec - 1) : 0);