minor bugfix: Correctly dereference "subdirname" variable

cppcheck reported:
[firejail/main.c:1335]: (warning) Char literal compared with pointer 'subdirname'. Did you intend to dereference it?

1 files changed, 1 insertions, 1 deletions

diff --git a/src/firejail/main.c b/src/firejail/main.c
index 012b2f230..95c325f9f 100644
--- a/src/firejail/main.c
+++ b/src/firejail/main.c
@@ -1332,7 +1332,7 @@ int main(int argc, char **argv) {
                                 arg_overlay_reuse = 1;
 
                                 char *subdirname = argv[i] + 16;
-                                if (subdirname == '\0') {
+                                if (*subdirname == '\0') {
                                         fprintf(stderr, "Error: invalid overlay option\n");
                                         exit(1);
                                 }