Fix segfault in log_internal if prefix is APK_OUT_LOG_ONLY

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.