Do not call abandon texture if there is no texture

Check if the texture was created before abandoning on SpriteAtlas
destructor. Linux driver was fine with that, but was crashing on Mac.

src/mbgl/geometry/sprite_atlas.cpp

@@ -299,6 +299,8 @@ void SpriteAtlas::bind(bool linear) {
 
 SpriteAtlas::~SpriteAtlas() {
     std::lock_guard<std::recursive_mutex> lock(mtx);
-    Environment::Get().abandonTexture(texture);
-    texture = 0;
+    if (texture) {
+        Environment::Get().abandonTexture(texture);
+        texture = 0;
+    }
 }

test/resources/resource_loader.cpp

@@ -59,8 +59,6 @@ public:
         spriteAtlas_.reset();
         glyphAtlas_.reset();
         glyphStore_.reset();
-
-        env_.performCleanup();
     }
 
     void update() {