ggml : fix null backend dereference (#4807)

* ggml : fix null backend dereference

* ggml : also check ggml_backend_is_cpu

Author: ggerganov

ggml-backend.c

@@ -703,7 +703,7 @@ ggml_backend_t ggml_backend_cpu_init(void) {
 }
 
 bool ggml_backend_is_cpu(ggml_backend_t backend) {
-    return backend->iface.get_name == ggml_backend_cpu_name;
+    return backend && backend->iface.get_name == ggml_backend_cpu_name;
 }
 
 void ggml_backend_cpu_set_n_threads(ggml_backend_t backend_cpu, int n_threads) {

ggml-cuda.cu

@@ -10140,7 +10140,7 @@ ggml_backend_t ggml_backend_cuda_init(int device) {
 }
 
 bool ggml_backend_is_cuda(ggml_backend_t backend) {
-    return backend->iface.get_name == ggml_backend_cuda_name;
+    return backend && backend->iface.get_name == ggml_backend_cuda_name;
 }
 
 int ggml_backend_cuda_get_device_count() {

ggml-metal.m

@@ -2749,7 +2749,7 @@ ggml_backend_t ggml_backend_metal_init(void) {
 }
 
 bool ggml_backend_is_metal(ggml_backend_t backend) {
-    return backend->iface.get_name == ggml_backend_metal_name;
+    return backend && backend->iface.get_name == ggml_backend_metal_name;
 }
 
 void ggml_backend_metal_set_n_cb(ggml_backend_t backend, int n_cb) {