Fix missing type checks before specbind

Fix missing type checks before specbind

This fixes bugs that crashed Emacs when the Lisp interpreter was fed
bad code.

* src/eval.c (FletX, Flet, internal_lisp_condition_case)
(funcall_lambda): Hoist symbol-with-pos elimination and type checks to a
dominating position for efficiency.  This also plugs at least two typing
holes. (Mea culpa.)
* test/src/eval-tests.el (eval-bad-specbind): New regression test.