Adding assert on malloc

We do this in other places and I think it's good behaviour for the
exceptional occasion that we cannot allocate memory.

src/Textures.cpp

@@ -999,6 +999,7 @@ void TextureCache::_load(u32 _tile, CachedTexture *_pTexture)
 	}
 
 	pDest = (u32*)malloc(_pTexture->textureBytes);
+	assert(pDest != NULL);
 
 	GLint mipLevel = 0, maxLevel = 0;
 #ifndef GLES2