Fix assert in c-exp.y

Fix assert in c-exp.y

The "restrict" patch added some asserts to c-exp.y, but one spot was
copy-pasted and referred to the wrong table.  This was pointed out by
-fsanitize=address.  This patch fixes the bug.

gdb/ChangeLog
2020-03-20  Tom Tromey  <tromey@adacore.com>

	* c-exp.y (lex_one_token): Fix assert.