* server/util_expr_eval.c (ap_expr_eval_re_backref): Fix gcc 7.x warning.

util_expr_eval.c: In function ‘ap_expr_eval_re_backref’:
util_expr_eval.c:265:63: error: comparison between pointer and zero character constant [-Werror=pointer-compare]
     if (!ctx->re_pmatch || !ctx->re_source || *ctx->re_source == '\0' ||


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1812307 13f79535-47bb-0310-9956-ffa450edef68