null check for getpwuid and pw_name

i3lock.c

@@ -661,7 +661,8 @@ static void raise_loop(xcb_window_t window) {
 }
 
 int main(int argc, char *argv[]) {
-    char *username = getpwuid(getuid())->pw_name;
+    struct passwd *pw = getpwuid(getuid());
+    char *username;
     char *image_path = NULL;
     int ret;
     struct pam_conv conv = {conv_callback, NULL};
@@ -686,8 +687,10 @@ int main(int argc, char *argv[]) {
         {NULL, no_argument, NULL, 0}
     };
 
-    if (username == NULL)
+    if (pw == NULL)
         err(EXIT_FAILURE, "getpwuid() failed");
+    if ((username = pw->pw_name) == NULL)
+        errx(EXIT_FAILURE, "pw->pw_name is NULL.\n");
 
     char *optstring = "hvnbdc:p:ui:teI:f";
     while ((o = getopt_long(argc, argv, optstring, longopts, &optind)) != -1) {