Fixed ngx_parse_time() out of bounds access (ticket #821).

The code failed to ensure that "s" is within the buffer passed for
parsing when checking for "ms", and this resulted in unexpected errors when
parsing non-null-terminated strings with trailing "m".  The bug manifested
itself when the expires directive was used with variables.

Found by Roman Arutyunyan.

src/core/ngx_parse.c

diff --git a/src/core/ngx_parse.c b/src/core/ngx_parse.c
index d7350d4..7b60c5f 100644
--- a/src/core/ngx_parse.c
+++ b/src/core/ngx_parse.c
@@ -188,7 +188,7 @@
             break;
 
         case 'm':
-            if (*p == 's') {
+            if (p < last && *p == 's') {
                 if (is_sec || step >= st_msec) {
                     return NGX_ERROR;
                 }