metal : fix failure to load model

The number of buffers in the ggml context was left unitialized.
This leads to sporadic failures to load the model on
startup. It is actually strange that the failure occurred so
infrequantly.

ggml-metal.m

@@ -86,6 +86,7 @@ struct ggml_metal_context * ggml_metal_init(void) {
 
     ctx->device = MTLCreateSystemDefaultDevice();
     ctx->queue  = [ctx->device newCommandQueue];
+    ctx->n_buffers = 0;
 
     // determine if we can use MPS
     if (MPSSupportsMTLDevice(ctx->device)) {