Test that symbolic links can end in '/'

This is obscure, but it is harmless.  It simply means that the target of
the symlink must be a directory (or symlink to a directory), otherwise
opening the symlink will fail with ENOTDIR.  Allowing non-symlink paths
to end in '/' is also harmless: opendir_safe() will set the last path
component to the empty string, and attempting to create a file, symlink,
or directory with the empty string as a name will fail with ENOENT.

(cherry picked from commit 9f78bc4)

qrexec-lib/unicode.c

@@ -137,7 +137,7 @@ static ssize_t validate_path(const uint8_t *const untrusted_name, size_t allowed
         if (i == 0 || untrusted_name[i - 1] == '/') {
             switch (untrusted_name[i]) {
             case '/': // repeated or initial slash
-            case '\0': // trailing slash or empty string
+            case '\0': // empty string
                 return -1;
             case '.':
                 if (untrusted_name[i + 1] == '\0' || untrusted_name[i + 1] == '/')

qrexec-lib/validator-test.c

@@ -219,4 +219,6 @@ int main(int argc, char **argv)
     assert(!qubes_pure_validate_symbolic_link((const uint8_t *)"a/b/c", (const uint8_t *)"/a"));
     // Symlinks may end in "..".
     assert(qubes_pure_validate_symbolic_link((const uint8_t *)"a/b/c", (const uint8_t *)".."));
+    // Symlinks may end in "/".
+    assert(qubes_pure_validate_symbolic_link((const uint8_t *)"a/b/c", (const uint8_t *)"a/"));
 }