Revise phone number length limit in registration view.

// FREEBIE

Signal/src/ViewControllers/ViewControllerUtils.m

@@ -49,10 +49,15 @@ NS_ASSUME_NONNULL_BEGIN
     NSString *center = insertionText.digitsOnly;
     // 4. Construct the "raw" new text by concatenating left, center and right.
     NSString *textAfterChange = [[left stringByAppendingString:center] stringByAppendingString:right];
-    // NOTE: We can't ensure that the number does not exceed the maximum length for a e164 phone number,
-    //       15 digits, per: https://en.wikipedia.org/wiki/E.164
-    //       because "valid numbers in Germany have been assigned that are longer than this" per:
+    // 4a. Ensure we don't exceed the maximum length for a e164 phone number,
+    //     15 digits, per: https://en.wikipedia.org/wiki/E.164
+    //
+    // NOTE: The actual limit is 18, not 15, because of certain invalid phone numbers in Germany.
     //       https://github.com/googlei18n/libphonenumber/blob/master/FALSEHOODS.md
+    const int kMaxPhoneNumberLength = 18;
+    if (textAfterChange.length > kMaxPhoneNumberLength) {
+        textAfterChange = [textAfterChange substringToIndex:kMaxPhoneNumberLength];
+    }
     // 5. Construct the "formatted" new text by inserting a hyphen if necessary.
     // reformat the phone number, trying to keep the cursor beside the inserted or deleted digit
     bool isJustDeletion = insertionText.length == 0;