cache: don't leave cache_slot fields uninitialized

Valgrind says:

==18344== Conditional jump or move depends on uninitialised value(s)
==18344==    at 0x406C83: open_slot (cache.c:63)
==18344==    by 0x407478: cache_ls (cache.c:403)
==18344==    by 0x404C9A: process_request (cgit.c:639)
==18344==    by 0x406BD2: fill_slot (cache.c:190)
==18344==    by 0x4071A0: cache_process (cache.c:284)
==18344==    by 0x404461: main (cgit.c:952)
==18344==  Uninitialised value was created by a stack allocation
==18344==    at 0x40738B: cache_ls (cache.c:375)

This is caused by the keylen field being used to calculate whether or
not a slot is matched.  We never then check the value of this and the
length of data read depends on the key length read from the file so this
isn't dangerous, but it's nice to avoid branching based on uninitialized
data.

Signed-off-by: John Keeping <john@keeping.me.uk>
tests: add Valgrind support

Now running tests with the "--valgrind" option will run cgit under
Valgrind instead of all Git commands.

Signed-off-by: John Keeping <john@keeping.me.uk>