ed: replace magic number (077776, i.e. 32766) with NBLK-1. (#300)

Temp file size is now declared in an enum; changing it from the
default introduces a subtle bug in putline(), which expects it to
be 32767.

Mask with NBLK-1 instead.

src/cmd/ed.c

@@ -1050,7 +1050,7 @@ putline(void)
 		}
 	}
 	nl = tline;
-	tline += ((lp-linebuf) + 03) & 077776;
+	tline += ((lp-linebuf) + 03) & (NBLK-1);
 	return nl;
 }