Merge branch 'bn/send-email-smtp-auth-error-message-fix'

Fix a minor regression brought in to "git send-email" by a recent
addition of the "--smtp-auth" option.

* bn/send-email-smtp-auth-error-message-fix:
  send-email: fix uninitialized var warning for $smtp_auth

git-send-email.perl

@@ -1136,7 +1136,7 @@ sub smtp_auth_maybe {
 
 	# Check mechanism naming as defined in:
 	# https://tools.ietf.org/html/rfc4422#page-8
-	if ($smtp_auth !~ /^(\b[A-Z0-9-_]{1,20}\s*)*$/) {
+	if ($smtp_auth && $smtp_auth !~ /^(\b[A-Z0-9-_]{1,20}\s*)*$/) {
 		die "invalid smtp auth: '${smtp_auth}'";
 	}