[PATCH] send-email: fix uninitialized var warning for $smtp_auth

On the latest version of git-send-email, I see this error just before
running SMTP auth (I didn't provide any --smtp-auth= parameter):

  Use of uninitialized value $smtp_auth in pattern match (m//) at /usr/local/google/home/briannorris/git/git/git-send-email.perl line 1139.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Jan Viktorin <redacted>
Cc: Eric Sunshine <redacted>
Cc: Junio C Hamano <redacted>
---
 git-send-email.perl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/git-send-email.perl b/git-send-email.perl
index c5a3f766f7fd..e3ff44b4d0cd 100755
--- a/git-send-email.perl
+++ b/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}'";
 	}
 

-- 
2.6.0.rc0.131.gf624c3d