Fix RTDB crash from race (#9123)

Author: paulb777

FirebaseDatabase/CHANGELOG.md

@@ -1,3 +1,6 @@
+# v8.11.0
+- [fixed] Race condition crash in FUtilities.m. (#9096)
+
 # v8.10.0
 - [fixed] Fixed URL handling bug when path is a substring of host. (#8874)
 

FirebaseDatabase/Sources/Utilities/FUtilities.m

@@ -295,10 +295,11 @@ + (NSError *)errorForStatus:(NSString *)status andReason:(NSString *)reason {
 }
 
 + (NSNumber *)intForString:(NSString *)string {
-    static NSCharacterSet *notDigits = nil;
-    if (!notDigits) {
-        notDigits = [[NSCharacterSet decimalDigitCharacterSet] invertedSet];
-    }
+    static dispatch_once_t once;
+    static NSCharacterSet *notDigits;
+    dispatch_once(&once, ^{
+      notDigits = [[NSCharacterSet decimalDigitCharacterSet] invertedSet];
+    });
     if ([string rangeOfCharacterFromSet:notDigits].length == 0) {
         NSInteger num;
         NSScanner *scanner = [NSScanner scannerWithString:string];