Metal: fix null pointer use in LatteTextureViewMtl destructor

std::map::operator[] inserts a default-constructed (null) value when
the key is absent. In GetSwizzledView, m_fallbackViewCache[key] would
insert a null entry, but if a free slot existed in m_viewCache the
texture was stored there instead, leaving the null entry in
m_fallbackViewCache. The destructor then iterated the map and called
->release() on that null pointer.

src/Cafe/HW/Latte/Renderer/Metal/LatteTextureViewMtl.cpp

@@ -104,17 +104,17 @@ MTL::Texture* LatteTextureViewMtl::GetSwizzledView(uint32 gpuSamplerSwizzle)
     }
 
     // Fallback cache
-    auto& fallbackEntry = m_fallbackViewCache[gpuSamplerSwizzle];
-    if (fallbackEntry)
+    auto it = m_fallbackViewCache.find(gpuSamplerSwizzle);
+    if (it != m_fallbackViewCache.end())
     {
-        return fallbackEntry;
+        return it->second;
     }
 
     MTL::Texture* texture = CreateSwizzledView(gpuSamplerSwizzle);
     if (freeIndex != -1)
         m_viewCache[freeIndex] = {gpuSamplerSwizzle, texture};
     else
-        fallbackEntry = texture;
+        m_fallbackViewCache[gpuSamplerSwizzle] = texture;
 
     return texture;
 }