http-push: fix construction of hex value from path

http-push: fix construction of hex value from path

The get_oid_hex_from_objpath takes care of creating a oid from a
pathname.  It does this by memcpy'ing the first two bytes of the path to
the "hex" string, then skipping the '/', and then copying the rest of the
path to the "hex" string.  Currently it fails to increase the pointer to
the hex string, so the second memcpy invocation just mashes over what
was copied in the first one, and leaves the last two bytes in the string
uninitialized.

This breaks valgrind in t5540, although the test passes without
valgrind:

==5490== Use of uninitialised value of size 8
==5490==    at 0x13C6B5: hexval (cache.h:1238)
==5490==    by 0x13C6DB: hex2chr (cache.h:1247)
==5490==    by 0x13C734: get_sha1_hex (hex.c:42)
==5490==    by 0x13C78E: get_oid_hex (hex.c:53)
==5490==    by 0x118BDA: get_oid_hex_from_objpath (http-push.c:1023)
==5490==    by 0x118C92: process_ls_object (http-push.c:1038)
==5490==    by 0x118E5B: handle_remote_ls_ctx (http-push.c:1077)
==5490==    by 0x118227: xml_end_tag (http-push.c:815)
==5490==    by 0x50C1448: ??? (in /usr/lib/libexpat.so.1.6.6)
==5490==    by 0x50C221B: ??? (in /usr/lib/libexpat.so.1.6.6)
==5490==    by 0x50BFBF2: ??? (in /usr/lib/libexpat.so.1.6.6)
==5490==    by 0x50C0B24: ??? (in /usr/lib/libexpat.so.1.6.6)
==5490==  Uninitialised value was created by a stack allocation
==5490==    at 0x118B63: get_oid_hex_from_objpath (http-push.c:1012)
==5490==

Fix this by correctly incrementing the pointer to the "hex" variable, so
the first two bytes are left untouched by the memcpy call, and the last
two bytes are correctly initialized.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>