Fixed segfault with try_files introduced by c985d90a8d1f.

If alias was used in a location given by a regular expression,
nginx used to do wrong thing in try_files if a location name (i.e.,
regular expression) was an exact prefix of URI.  The following
configuration triggered a segmentation fault on a request to "/mail":

    location ~ /mail {
        alias /path/to/directory;
        try_files $uri =404;
    }

Reported by Per Hansson.

src/http/ngx_http_core_module.c

diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index d423d8b..8c8d23c 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -1239,7 +1239,9 @@
 
             *e.pos = '\0';
 
-            if (alias && ngx_strncmp(name, clcf->name.data, alias) == 0) {
+            if (alias && alias != NGX_MAX_SIZE_T_VALUE
+                && ngx_strncmp(name, clcf->name.data, alias) == 0)
+            {
                 ngx_memmove(name, name + alias, len - alias);
                 path.len -= alias;
             }