Core: strengthen configuration syntax checker.

It is now a syntax error if tokens passed to a custom configuration
handler are terminated by "{".

The following incorrect configuration is now properly rejected:

  map $v $v2 {
      a b {
      c d {
      e f {
  }

src/core/ngx_conf_file.c

diff --git a/src/core/ngx_conf_file.c b/src/core/ngx_conf_file.c
index a262672..0d12ec2 100644
--- a/src/core/ngx_conf_file.c
+++ b/src/core/ngx_conf_file.c
@@ -225,6 +225,11 @@
              * "types { ... }" directive
              */
 
+            if (rc == NGX_CONF_BLOCK_START) {
+                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "unexpected \"{\"");
+                goto failed;
+            }
+
             rv = (*cf->handler)(cf, NULL, cf->handler_conf);
             if (rv == NGX_CONF_OK) {
                 continue;