dd: Fix backwards comparison when handling short writes

ipos is always ahead of opos, so the left side of this condition
was never true. This only mattered when we get short writes, since
on EOF we always have less than a full output block, so it takes
only one normal write.

dd.c

@@ -221,7 +221,7 @@ main(int argc, char *argv[])
 			else
 				ofull++;
 			opos += ret;
-		} while ((eof && ipos < opos) || (!eof && ipos - opos >= obs));
+		} while (ipos - opos >= (eof ? 1 : obs));
 		if (opos < ipos)
 			memmove(buf, buf + opos, ipos - opos);
 		ipos -= opos;