don't let trailing spaces break filters.

Reference: /n/sources/patch/applied/snoopy-trailingspace
Date: Thu Aug 16 17:44:55 CES 2007
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/ip/snoopy/filter.y	Thu Aug 16 17:44:24 2007
+++ /sys/src/cmd/ip/snoopy/filter.y	Thu Aug 16 17:44:22 2007
@@ -64,10 +64,12 @@
 	char *p;
 	int c;
 
-	if(yylp == nil || *yylp == 0)
+	if(yylp == nil)
 		return 0;
 	while(isspace(*yylp))
 		yylp++;
+	if(*yylp == 0)
+		return 0;
 
 	yylval = newfilter();