Add an eassert back to XSYMBOL

Add an eassert back to XSYMBOL

Problem reported by Alan Mackenzie in:
https://lists.gnu.org/r/emacs-devel/2024-01/msg00755.html
* src/lisp.h (XSYMBOL): If the arg is not a bare symbol, then
eassert (symbols_with_pos_enabled).  This shouldn’t affect code
generated for regular builds, and could catch caller errors in
debug builds.  For debug builds although this slows things down
XSYMBOL should still be faster than it was the day before
yesterday, as there’s still no need to eassert (SYMBOLP	(a)).