* doprnt.c: Prefer signed to unsigned when either works.

* doprnt.c: Prefer signed to unsigned when either works.

* eval.c (verror):
* doprnt.c (doprnt):
* lisp.h (doprnt):
* xdisp.c (vmessage):
Use ptrdiff_t, not size_t, when using or implementing doprnt,
since the sizes cannot exceed ptrdiff_t bounds anyway, and we
prefer signed arithmetic to avoid comparison confusion.
* doprnt.c (doprnt): Avoid a "+ 1" that can't overflow,
but is a bit tricky.