Fix merging of files that lack newlines for diff(3), OpenRCS, and OpenCVS.

Merges with a file that lacks newlines (\n) were triggering a fatal error.
This could be easily reproduced with merge(1) and diff3(1):
$ echo foo > foo
$ echo bar > bar
$ echo -n baz > baz
$ merge -p foo bar baz
merge: failed to merge
$ diff3 -E foo bar baz
1a
=======
diff3prog: logic error
$

Fix this by properly handling short reads from the third file argument.
Only the third file argument triggered the problem. The other input
files were already handled correctly.

ok millert@

1 files changed, 9 insertions, 2 deletions

diff --git a/usr.bin/diff3/diff3prog.c b/usr.bin/diff3/diff3prog.c
index 992037b6d85..a56bfc7ce7a 100644
--- a/usr.bin/diff3/diff3prog.c
+++ b/usr.bin/diff3/diff3prog.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: diff3prog.c,v 1.19 2016/10/18 21:06:52 millert Exp $	*/
+/*	$OpenBSD: diff3prog.c,v 1.20 2020/06/26 07:28:47 stsp Exp $	*/
 
 /*
  * Copyright (C) Caldera International Inc.  2001-2002.
@@ -555,9 +555,16 @@ edscript(int n)
 			printf("%da\n=======\n", de[n].old.to -1);
 		(void)fseek(fp[2], (long)de[n].new.from, SEEK_SET);
 		for (k = de[n].new.to-de[n].new.from; k > 0; k-= j) {
+			size_t r;
 			j = k > BUFSIZ ? BUFSIZ : k;
-			if (fread(block, 1, j, fp[2]) != j)
+			r = fread(block, 1, j, fp[2]);
+			if (r == 0) {
+				if (feof(fp[2]))
+					break;
 				trouble();
+			}
+			if (r != j)
+				j = r;
 			(void)fwrite(block, 1, j, stdout);
 		}
 		if (!oflag || !overlap[n])