Fixes a recursion bug introduced in the previous diff

Summary:
Reverting the portion of the previous diff that attempted to reuse a reference as a bool,
given that the reference is assigned to late to be used to short-circuit recursion

src/Facebook.h

@@ -36,6 +36,7 @@
     NSString* _appId;
     NSString* _urlSchemeSuffix;
     NSArray* _permissions;
+    BOOL _isExtendingAccessToken;
     FBRequest *_requestExtendingAccessToken;
     NSDate* _lastAccessTokenUpdate;
     FBFrictionlessRequestSettings* _frictionlessRequestSettings;

src/Facebook.m

@@ -347,9 +347,10 @@ - (void)authorize:(NSArray *)permissions {
  * applicationDidBecomeActive: UIApplicationDelegate method.
  */
 - (void)extendAccessToken {
-    if (_requestExtendingAccessToken) {
+    if (_isExtendingAccessToken) {
         return;
     }
+    _isExtendingAccessToken = YES;
     NSMutableDictionary* params = [NSMutableDictionary dictionaryWithObjectsAndKeys:
                                    @"auth.extendSSOAccessToken", @"method",
                                    nil];
@@ -805,10 +806,12 @@ - (void)fbDialogNotLogin:(BOOL)cancelled {
 // These delegate methods are only called for requests that extendAccessToken initiated
 
 - (void)request:(FBRequest *)request didFailWithError:(NSError *)error {
+    _isExtendingAccessToken = NO;
     _requestExtendingAccessToken = nil;
 }
 
 - (void)request:(FBRequest *)request didLoad:(id)result {
+    _isExtendingAccessToken = NO;
     _requestExtendingAccessToken = nil;
     NSString* accessToken = [result objectForKey:@"access_token"];
     NSString* expTime = [result objectForKey:@"expires_at"];