Fix suicide in vacfs when no prefix specified.
It has been reviewed and merged in plan9port.

Reference: /n/sources/patch/applied/vacfs-prefix
Date: Thu Dec  1 09:38:15 CET 2011
Signed-off-by: djc@9grid.fr

--- /sys/src/cmd/vac/file.c	Thu Dec  1 09:38:03 2011
+++ /sys/src/cmd/vac/file.c	Thu Dec  1 09:37:55 2011
@@ -1774,7 +1774,7 @@
 	char *prefix;
 	
 	if(vtparsescore(file, &prefix, score) >= 0){
-		if(strcmp(prefix, "vac") != 0){
+		if(prefix == nil || strcmp(prefix, "vac") != 0){
 			werrstr("not a vac file");
 			return nil;
 		}