Fix some unsafe uses of SSDATA in comp.c

Fix some unsafe uses of SSDATA in comp.c

* src/comp.c (comp_hash_source_file)
(Fcomp__compile_ctxt_to_file, Fnative_elisp_load): Encode file
names before passing them to library APIs.
(Fcomp__compile_ctxt_to_file): use emacs_fopen instead of fopen.
(declare_lex_function): Avoid keeping a 'char *' pointer around
while calling Lisp, which could trigger GC, which could relocate
string data.