[ios] Fixed infinite loop zooming in past z23

At zoom levels where the minimum 1 meter or 4 feet would be wider than the scale bar’s maximum width, the local variable holding the preferred row was left undefined. A loop that later iterated based on this row would effectively iterate infinitely until memory pressure forces the system to quit the application.

platform/ios/src/MGLScaleBar.mm

@@ -188,9 +188,9 @@ static const CGFloat MGLFeetPerMeter = 3.28084;
 
 - (MGLRow)preferredRow {
     CLLocationDistance maximumDistance = [self maximumWidth] * [self unitsPerPoint];
-    MGLRow row;
     
     BOOL useMetric = [self usesMetricSystem];
+    MGLRow row = useMetric ? MGLMetricTable[0] : MGLImperialTable[0];
     NSUInteger count = useMetric
     ? sizeof(MGLMetricTable) / sizeof(MGLMetricTable[0])
     : sizeof(MGLImperialTable) / sizeof(MGLImperialTable[0]);