Clear static_links in reread_symbols

Clear static_links in reread_symbols

-fsanitize=address pointed out a use-after free in
objfile_register_static_link.  The bug turned out to be that
reread_symbols does not clear the static_links field; this leaves a
hash table that is filled with freed pointers.

Jan's (now quite old) idea of replacing reread_symbols with a simple
delete/new still seems good to me, and it's worth noting that it would
have avoided this bug.

Tested by the buildbot and by observing the change with
-fsanitize=address.

gdb/ChangeLog
2018-10-19  Tom Tromey  <tom@tromey.com>

	* symfile.c (reread_symbols): Clear "static_links".