This commit fixes a regression which was introduced in changeset
646c8344. If apk_out_fmt() is called
while out->log is set and prefix is set to APK_OUT_LOG_ONLY, then
apk_out_fmt() would pass this prefix to log_internal() which would, in
turn, attempt to write it to standard out using fprintf().
Unfortunately, doing so wont work as intended if prefix is ((char*)-1)
(i.e. APK_OUT_LOG_ONLY) and will cause a segmentation fault instead.

This commit fixes this segmentation fault by not printing the prefix in
log_internal() if it is either NULL or APK_OUT_LOG_ONLY.