Avoid casting -1 to possibly-unsigned enum

Avoid casting -1 to possibly-unsigned enum

* src/alloc.c (mark_maybe_pointer):
* src/pdumper.h (pdumper_object_p_precise):
Use pdumper_valid_object_type_p.
* src/pdumper.c (pdumper_find_object_type_impl):
* src/pdumper.h (pdumper_find_object_type):
Return int, not enum Lisp_Type.  All callers changed.
* src/pdumper.h (PDUMPER_NO_OBJECT): Do not cast -1 to enum
Lisp_Type; in theory, C18 says this could yield 7, which would
mean PDUMPER_NO_OBJECT == Lisp_Float (!).
(pdumper_valid_object_type_p): New function.