Improve error messages for `$' or `@' followed by `.' or `-'.

Improve error messages for `$' or `@' followed by `.' or `-'.

Previously, for this special case of an invalid reference, the
usual "symbol not found in production:" was printed.  However,
because the symbol name was parsed as the empty string, that
message was followed immediately by a newline instead of a symbol
name.  In reality, this is a syntax error, so the reference is
invalid regardless of the symbols actually appearing in the
production.  Discussed at
<http://lists.gnu.org/archive/html/bison-patches/2011-01/msg00012.html>.
* src/scan-code.l (parse_ref): Report the above case as a syntax
error.  Other than that, continue to handle this case like any
other invalid reference that Bison manages to parse because
"possibly meant" messages can still be helpful to the user.
* tests/named-refs.at ($ or @ followed by . or -): New test group.