bison: avoid warnings from static code analysis

bison: avoid warnings from static code analysis

A static analysis tool reports that some callers of symbol_list_n_get
might get NULL and not handle it properly.  This is not the case, yet
we can suppress this pattern.

Reported by Mike Sullivan.
<https://lists.gnu.org/archive/html/bug-bison/2013-12/msg00027.html>

* src/symlist.c (symbol_list_n_get): Actually it is never called
to return 0.  Enforce this postcondition via aver.
(symbol_list_n_type_name_get): Simplify accordingly.  In particular,
discards a (translated) useless error message.
* src/symlist.h: Adjust documentation.
* src/scan-code.l: Style change.