pmap: Fix extended mode in one_proc().

pmap: Fix extended mode in one_proc().

Check the return value of sscanf() to make sure that all input items are
properly initialized.

In extended mode (x_option), one_proc() loads the values of start and
perms during one iteration of the while loop, and displays them during
one of the following iterations, but start and perms are variables local
to the while loop: move them out of the while loop, to the beginning of
the function.

Also, display a mapping only if cp2 is properly initialized; otherwise
(for example), mappings that do not belong to a selected range are
displayed, and with a NULL mapping name:

$ pmap -x -A 6FFF00000000,7FFF00000000 $$
...
Address           Kbytes     RSS   Dirty Mode  Mapping
000055b3d1e9b000       0     912       0  r-xp (null)
000055b3d2194000       0      16      16  r--p (null)
000055b3d2198000       0      36      36  rw-p (null)
...