metal: check MTLBuffer allocations for nil before using their contents.

(cherry picked from commit e1b3c8afd574e9da9638103646792d75bfee7394)

src/render/metal/SDL_render_metal.m

@@ -1564,6 +1564,9 @@ static bool SetDrawState(SDL_Renderer *renderer, const SDL_RenderCommand *cmd, c
     if (statecache->shader_constants_dirty ||
         SDL_memcmp(shader_constants, &statecache->shader_constants, sizeof(*shader_constants)) != 0) {
         id<MTLBuffer> mtlbufconstants = [data.mtldevice newBufferWithLength:sizeof(*shader_constants) options:MTLResourceStorageModeShared];
+        if (mtlbufconstants == nil) {
+            return SDL_OutOfMemory();
+        }
         mtlbufconstants.label = @"SDL shader constants data";
         SDL_memcpy([mtlbufconstants contents], shader_constants, sizeof(*shader_constants));
         [data.mtlcmdencoder setFragmentBuffer:mtlbufconstants offset:0 atIndex:0];
@@ -1734,6 +1737,9 @@ static bool METAL_RunCommandQueue(SDL_Renderer *renderer, SDL_RenderCommand *cmd
              * practices guide recommends this approach for streamed vertex data.
              */
             mtlbufvertex = [data.mtldevice newBufferWithLength:vertsize options:MTLResourceStorageModeShared];
+            if (mtlbufvertex == nil) {
+                return SDL_OutOfMemory();
+            }
             mtlbufvertex.label = @"SDL vertex data";
             SDL_memcpy([mtlbufvertex contents], vertices, vertsize);