hmp: expr_unary(): check for overflow in strtoul()/strtoull()

It's not checked currently, so something like:

  (qemu) balloon -100000000000001111114334234
  (qemu)

Will just "work" (in this case the balloon command will get a random
value).

Fix it by checking if strtoul()/strtoull() overflowed.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
