bug fixed by nemo in Nix: in one place cpp was checking open for <= 0
as an error. fix: check < 0 only

Reference: /n/sources/patch/applied/cpp-open
Date: Fri Apr 20 17:01:46 CES 2012
Signed-off-by: forsyth@terzarima.net

--- /sys/src/cmd/cpp/nlist.c	Fri Apr 20 17:01:04 2012
+++ /sys/src/cmd/cpp/nlist.c	Fri Apr 20 17:01:02 2012
@@ -170,7 +170,7 @@
 			dp[len] = '\0';
 		}
 		fp = (char*)newstring((uchar*)argv[0], strlen(argv[0]), 0);
-		if ((fd = open(fp, 0)) <= 0)
+		if ((fd = open(fp, 0)) < 0)
 			error(FATAL, "Can't open input file %s", fp);
 	}
 	if (argc > 1) {