fixes #238: don't crash on CGRectZero/invalid frame with MapBox sources

@@ -37,6 +37,7 @@
     if (self.mapScrollViewDelegate)
         [self.mapScrollViewDelegate scrollView:self correctedContentOffset:&contentOffset];
 
+    if (contentOffset.x && contentOffset.y)
         [super setContentOffset:contentOffset];
 }
 
@@ -45,6 +46,7 @@
     if (self.mapScrollViewDelegate)
         [self.mapScrollViewDelegate scrollView:self correctedContentOffset:&contentOffset];
 
+    if (contentOffset.x && contentOffset.y)
         [super setContentOffset:contentOffset animated:animated];
 }