SSL: overcame possible buffer over-read in ngx_ssl_error().

It appeared that ERR_error_string_n() cannot handle zero buffer size well enough
and causes over-read.

The problem has also been fixed in OpenSSL:
https://git.openssl.org/?p=openssl.git;h=e5c1361580d8de79682958b04a5f0d262e680f8b

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 68d02bf..cddcefd 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -2137,7 +2137,9 @@
             break;
         }
 
-        if (p >= last) {
+        /* ERR_error_string_n() requires at least one byte */
+
+        if (p >= last - 1) {
             goto next;
         }