main: make sure the ambient caps set is valid in case we fail to read it

We ignore failures when reading this after all. Hence we better leave
the memory properly initialized.

src/core/main.c

@@ -3013,7 +3013,7 @@ int main(int argc, char *argv[]) {
         usec_t before_startup, after_startup;
         static char systemd[] = "systemd";
         const char *error_message = NULL;
-        uint64_t saved_ambient_set;
+        uint64_t saved_ambient_set = 0;
         int r, retval = EXIT_FAILURE;
         Manager *m = NULL;
         FDSet *fds = NULL;