Don't allow empty pathnames in fast-import

riddochc on #git noticed corruption caused by import-tars.  This
was fixed in the prior commit by Dscho, but fast-import was wrong
to have allowed a tree to be created with an empty string as the
filename.  No operating system allows this, and Git itself doesn't
accept this into the index.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -1193,6 +1193,8 @@ static int tree_content_set(
 		n = slash1 - p;
 	else
 		n = strlen(p);
+	if (!n)
+		die("Empty path component found in input");
 
 	for (i = 0; i < t->entry_count; i++) {
 		e = t->entries[i];