hexdump: convert a variable type according with fmt

hexdump works uncorrectly on Rassberry Pi (raspbian wheezy):
0000000 3200000000 3400000000 3600000000 3800000000 a00000000
000000a

The problem is that the %qx format is used for printing
the (short int) variable.

Here is the output from hexdump with this patch:
0000000 3231 3433 3635 3837 0a39
000000a

Currently raspbian uses hexdump from bsdmainutils.
bsdmainutils: /usr/bin/hexdump

Signed-off-by: Andrew Vagin <avagin@openvz.org>
hexdump: initialize hex->skip

==11097== Conditional jump or move depends on uninitialised value(s)
==11097==    at 0x9588: next (hexdump-display.c:355)
==11097==    by 0x9767: display (hexdump-display.c:280)
==11097==    by 0x9227: main (hexdump.c:198)

Signed-off-by: Andrew Vagin <avagin@openvz.org>
hexdump: don't access hex after free

==20209== Invalid read of size 4
==20209==    at 0x923C: main (hexdump.c:202)
==20209==  Address 0x49af034 is 12 bytes inside a block of size 32 free'd
==20209==    at 0x48348B8: free (vg_replace_malloc.c:427)
==20209==    by 0x923B: main (hexdump.c:200)

Signed-off-by: Andrew Vagin <avagin@openvz.org>