SDK-562 add reference link. add safety check.

Author: echo-branch

Branch-SDK/Branch-SDK/BNCNetworkInterface.m

@@ -115,7 +115,9 @@ + (nullable NSString *)localIPAddress {
 + (NSArray<NSString *> *)allIPAddresses {
     NSMutableArray *array = [NSMutableArray new];
     for (BNCNetworkInterface *inf in [BNCNetworkInterface currentInterfaces]) {
-        [array addObject:inf.description];
+        if (inf.description) {
+            [array addObject:inf.description];
+        }
     }
     return array;
 }

Branch-SDK/Branch-SDK/BNCReachability.m

@@ -21,6 +21,7 @@ @interface BNCReachability()
 @end
 
 // Based on Apple's Reachability Sample
+// https://developer.apple.com/library/archive/samplecode/Reachability/Introduction/Intro.html
 @implementation BNCReachability
 
 - (instancetype)init {