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 #1152 (CVE-2018-18088).