py/builtinevex: Handle invalid filenames for execfile.

If a non-string buffer was passed to execfile, then it would be passed
as a non-null-terminated char* to mp_lexer_new_from_file.

This changes mp_lexer_new_from_file to take a qstr instead (as in almost
all cases a qstr will be created from this input anyway to set the
`__file__` attribute on the module).

This now makes execfile require a string (not generic buffer) argument,
which is probably a good fix to make anyway.

Fixes issue #12522.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/powerpc/main.c

@@ -111,7 +111,7 @@ void gc_collect(void) {
     gc_dump_info(&mp_plat_print);
 }
 
-mp_lexer_t *mp_lexer_new_from_file(const char *filename) {
+mp_lexer_t *mp_lexer_new_from_file(qstr filename) {
     mp_raise_OSError(MP_ENOENT);
 }