metal : null-check buffer alloc to fix OOM crash (#25371)

* metal : null-check ggml_metal_buffer_init result to avoid OOM crash

ggml_backend_metal_buffer_type_alloc_buffer used the result of
ggml_metal_buffer_init without checking for NULL. ggml_metal_buffer_init
returns NULL when the underlying Metal allocation fails (e.g. an
out-of-memory condition), and the following ggml_metal_buffer_is_shared(res)
call dereferences it, turning a recoverable allocation failure into a hard
crash (EXC_BAD_ACCESS). This is easy to hit on memory-constrained devices
such as iOS when a model/context exceeds the available Metal budget.

Log the failure using the existing GGML_LOG_ERROR convention and return
NULL so the allocator surfaces a diagnosable error up the stack instead of
crashing.

* cont : fix log

---------

Co-authored-by: Georgi Gerganov <[email protected]>

ggml/src/ggml-metal/ggml-metal.cpp

@@ -204,6 +204,11 @@ static ggml_backend_buffer_t ggml_backend_metal_buffer_type_alloc_buffer(ggml_ba
     ggml_metal_device_t ctx_dev = (ggml_metal_device_t)buft->device->context;
     ggml_metal_buffer_t res = ggml_metal_buffer_init(ctx_dev, size, shared);
 
+    if (res == NULL) {
+        GGML_LOG_ERROR("%s: failed to allocate Metal buffer of %zu bytes (out of memory)\n", __func__, size);
+        return NULL;
+    }
+
     ggml_backend_buffer_i buf_i = ggml_metal_buffer_is_shared(res)
         ? ggml_backend_metal_buffer_shared_i
         : ggml_backend_metal_buffer_private_i;