open_without_symlink: Account when prefix is empty string

In the current implementation, the open_without_symlink function
will default to opening the root mount only if the passed rootfs
prefix is null. It doesn't account for the case where this prefix
is passed as an empty string.

Properly handle this second case as well.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@nxp.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>

src/lxc/utils.c

@@ -1575,7 +1575,7 @@ static int open_without_symlink(const char *target, const char *prefix_skip)
 	fulllen = strlen(target);
 
 	/* make sure prefix-skip makes sense */
-	if (prefix_skip) {
+	if (prefix_skip && strlen(prefix_skip) > 0) {
 		curlen = strlen(prefix_skip);
 		if (!is_subdir(target, prefix_skip, curlen)) {
 			ERROR("WHOA there - target '%s' didn't start with prefix '%s'",