Fix a possible null pointer dereference in ap_expr_parse()

Fix a possible null pointer dereference in ap_expr_parse()

In ap_expr_parse(), ap_expr_yylex_init() will return 1 on failure,
and ctx.scanner will remain NULL. However the return value of
ap_expr_yylex_init() is not checked, and there is a dereference of
ctx.scanner in following function ap_expr_yyset_extra(),
which may lead to NULL pointer dereference.

Fix this bug by adding return value check of ap_expr_yylex_init.

Submitted by: Zhou Qingyang <zhou1615@umn.edu>

Github: closes #308

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1908772 13f79535-47bb-0310-9956-ffa450edef68