fix memory handling in acme-client config parser

[Jonathan Gray]

Use after free and a memory leak.

Index: parse.y
===================================================================
RCS file: /cvs/src/usr.sbin/acme-client/parse.y,v
retrieving revision 1.17
diff -u -p -U4 -r1.17 parse.y
--- parse.y     23 Mar 2017 12:59:32 -0000      1.17
+++ parse.y     19 Oct 2017 04:50:29 -0000
@@ -224,10 +224,10 @@ domain            : DOMAIN STRING {
                        char *s;
                        if ((s = strdup($2)) == NULL)
                                err(EXIT_FAILURE, "strdup");
                        if (!domain_valid(s)) {
-                               free(s);
                                yyerror("%s: bad domain syntax", s);
+                               free(s);
                                YYERROR;
                        }
                        if ((domain = conf_new_domain(conf, s)) == NULL) {
                                free(s);
@@ -335,8 +335,9 @@ domainoptsl : ALTERNATIVE NAMES '{' altn
                        if ((s = strdup($3)) == NULL)
                                err(EXIT_FAILURE, "strdup");
                        if (authority_find(conf, s) == NULL) {
                                yyerror("use: unknown authority");
+                               free(s);
                                YYERROR;
                        }
                        domain->auth = s;
                }