Fix use-after-free after a deferred alert.

The KLEE folks (who do symbolic execution of code) found a crash:
http://marc.info/?l=openssl-dev&m=139809493725682&w=2

ssl/s3_pkt.c

diff --git a/ssl/s3_pkt.c b/ssl/s3_pkt.c
index a41399b..ebea57a 100644
--- a/ssl/s3_pkt.c
+++ b/ssl/s3_pkt.c
@@ -678,6 +678,11 @@
 		/* if it went, fall through and send more stuff */
 		}
 
+	/* Dispatching the alert may have released the write buffer. */
+	if (wb->buf == NULL)
+		if (!ssl3_setup_write_buffer(s))
+			return -1;
+
 	if (len == 0)
 		return 0;