Prevent infinite loop in upas/pop3 when reading a message header
with a line more than 8kbytes long.

Reference: /n/sources/patch/applied/pop3-header-loop
Date: Mon May  2 15:10:14 CES 2005

--- /sys/src/cmd/upas/pop3/pop3.c	Mon May  2 15:08:40 2005
+++ /sys/src/cmd/upas/pop3/pop3.c	Mon May  2 15:08:37 2005
@@ -244,8 +244,9 @@
 		for(;;){
 			p = Brdline(b, '\n');
 			if(p == nil){
-				if(Blinelen(b) == 0)
+				if((n = Blinelen(b)) == 0)
 					break;
+				Bseek(b, n, 1);
 			}else
 				lines++;
 		}