closes #291: merge in parts of @RubioV/RMBingSource-offline-crash-fix

@@ -85,7 +85,11 @@
 
     NSArray *URLs = [self URLsForTile:tile];
 
-    if ([URLs count] > 1)
+    if ([URLs count] == 0)
+    {
+        return nil;
+    }
+    else if ([URLs count] > 1)
     {
         // fill up collection array with placeholders
         //

@@ -27,7 +27,7 @@
 
 #import "RMAbstractWebMapSource.h"
 
-typedef enum NSUInteger {
+typedef enum : NSUInteger {
     RMBingImagerySetRoad             = 0, // default
     RMBingImagerySetAerial           = 1,
     RMBingImagerySetAerialWithLabels = 2,

@@ -75,7 +75,10 @@
 
         NSData *metadataData = [NSData brandedDataWithContentsOfURL:metadataURL];
 
-        id metadata = [NSJSONSerialization JSONObjectWithData:metadataData options:0 error:nil];
+        if ( ! metadataData)
+            return nil;
+
+        id metadata = [NSJSONSerialization JSONObjectWithData:metadataData options:0 error:NULL];
 
         if (metadata && [metadata isKindOfClass:[NSDictionary class]] && [[metadata objectForKey:@"statusCode"] intValue] == 200)
         {