Metal: Fixed ObjC -> C string conversion.

src/renderer_mtl.h

@@ -141,7 +141,7 @@ namespace bgfx { namespace mtl
 			id<MTLLibrary> lib =  [m_obj newLibraryWithData:(dispatch_data_t)_data error:&error];
 			BX_WARN(NULL == error
 				, "newLibraryWithData failed: %s"
-				, error.localizedDescription.UTF8String
+				, [error.localizedDescription cStringUsingEncoding:NSASCIIStringEncoding]
 				);
 			return lib;
 		}
@@ -150,10 +150,10 @@ namespace bgfx { namespace mtl
 		{
 			NSError* error;
 			id<MTLLibrary> lib = [m_obj newLibraryWithSource:@(_source) options:nil error:&error];
-//			BX_WARN(NULL == error
-//				, "Shader compilation failed: %s"
-//				, [error.localizedDescription UTF8String]
-//				);
+			BX_WARN(NULL == error
+				, "Shader compilation failed: %s"
+				, [error.localizedDescription cStringUsingEncoding:NSASCIIStringEncoding]
+				);
 			return lib;
 		}
 
@@ -200,7 +200,7 @@ namespace bgfx { namespace mtl
 			id <MTLRenderPipelineState> state = [m_obj newRenderPipelineStateWithDescriptor:_descriptor error:&error];
 			BX_WARN(NULL == error
 				, "newRenderPipelineStateWithDescriptor failed: %s"
-				, error.localizedDescription.UTF8String
+				, [error.localizedDescription cStringUsingEncoding:NSASCIIStringEncoding]
 				);
 			return state;
 		}
@@ -212,7 +212,7 @@ namespace bgfx { namespace mtl
 
 			BX_WARN(NULL == error
 				, "newRenderPipelineStateWithDescriptor failed: %s"
-				, error.localizedDescription.UTF8String
+				, [error.localizedDescription cStringUsingEncoding:NSASCIIStringEncoding]
 				);
 			return state;
 		}
@@ -225,7 +225,7 @@ namespace bgfx { namespace mtl
 
 			BX_WARN(NULL == error
 				, "newComputePipelineStateWithFunction failed: %s"
-				, error.localizedDescription.UTF8String
+				, [error.localizedDescription cStringUsingEncoding:NSASCIIStringEncoding]
 				);
 			return state;
 		}