metal: fix regression when no metal devices are present

Otherwise, metal initialization segfaults.

This situation was fixed before in
acd38ef. Regression was introduced in
bf9087f.

Author: booxter

ggml/src/ggml-metal/ggml-metal.m

@@ -92,7 +92,10 @@
 
     if (ctx->mtl_device == nil) {
         ctx->mtl_device = MTLCreateSystemDefaultDevice();
+    }
 
+    // There may be still no Metal device available
+    if (ctx->mtl_device) {
         ctx->has_simdgroup_reduction  = [ctx->mtl_device supportsFamily:MTLGPUFamilyApple7];
         ctx->has_simdgroup_reduction |= [ctx->mtl_device supportsFamily:MTLGPUFamilyMetal3_GGML];