metal : fix failure to load model (ggml-org#1817)

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.

Co-authored-by: Iwan Kawrakow <[email protected]>

Author: ikawrakow

ggml-metal.m

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