tic crash with unresolved use

[Miroslav Lichvar]

This was reported by the clang static analyzer and confirmed with
the following test:

foo|description,
        use=nosuchterm, use=xterm

This causes a crash in _nc_resolve_uses2() in the new code recently
added to check for duplicate use.

A possible fix:

--- a/ncurses/tinfo/comp_parse.c
+++ b/ncurses/tinfo/comp_parse.c
@@ -457,7 +457,8 @@ _nc_resolve_uses2(bool fullresolve, bool literal)
 
                    /* verify that there are no earlier uses */
                    for (j = 0; j < i; ++j) {
-                       if (!strcmp(qp->uses[j].link->tterm.term_names,
+                       if (qp->uses[j].link &&
+                           !strcmp(qp->uses[j].link->tterm.term_names,
                                    rp->tterm.term_names)) {
                            _nc_warning("duplicate use=%s", lookfor);
                            break;
@@ -487,7 +488,8 @@ _nc_resolve_uses2(bool fullresolve, bool literal)
 
                    /* verify that there are no earlier uses */
                    for (j = 0; j < i; ++j) {
-                       if (!strcmp(qp->uses[j].link->tterm.term_names,
+                       if (qp->uses[j].link &&
+                           !strcmp(qp->uses[j].link->tterm.term_names,
                                    rp->tterm.term_names)) {
                            _nc_warning("duplicate use=%s", lookfor);
                            break;

-- 
Miroslav Lichvar