Fix the validation of main device (#1872)

ggml-cuda.cu

@@ -2366,7 +2366,7 @@ void ggml_cuda_assign_buffers_no_scratch(struct ggml_tensor * tensor) {
 }
 
 void ggml_cuda_set_main_device(int main_device) {
-    if (main_device > g_device_count) {
+    if (main_device >= g_device_count) {
         fprintf(stderr, "warning: cannot set main_device=%d because there are only %d devices. Using device %d instead.\n",
                 main_device, g_device_count, g_main_device);
         return;