vi: correctly detect when a deletion empties the buffer

Michał Berger has reported two issues:

- Repeatedly deleting and undoing the deletion of the last line
  results in characters being lost from the end of the line.

- Deleting the bottom line twice then attempting to undo each of
  these deletions results in a segfault.

The problem seems to be an incorrect test for whether the text buffer
is empty.

Reported-by: Michał Berger <michallinuxstuff@gmail.com>
Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

editors/vi.c

@@ -2364,7 +2364,7 @@ static void undo_push(char *src, unsigned int length, uint8_t u_type)	// Add to
 	// Allocate a new undo object
 	if (u_type == UNDO_DEL || u_type == UNDO_DEL_CHAIN) {
 		// For UNDO_DEL objects, save deleted text
-		if ((src + length) == end)
+		if ((text + length) == end)
 			length--;
 		// If this deletion empties text[], strip the newline. When the buffer becomes
 		// zero-length, a newline is added back, which requires this to compensate.