vircrypto: fix Invalid write in virCryptoHashString()

vircrypto: fix Invalid write in virCryptoHashString()

While running vircryptotest, it was found that valgrind pointed out the
following error:

==27453== Invalid write of size 1
==27453==    at 0x4C7D7C9: virCryptoHashString (vircrypto.c:76)
==27453==    by 0x401C4E: testCryptoHash (vircryptotest.c:41)
==27453==    by 0x402A11: virtTestRun (testutils.c:199)
==27453==    by 0x401AD5: mymain (vircryptotest.c:76)
==27453==    by 0x40318D: virtTestMain (testutils.c:782)
==27453==    by 0x3E6CE1ED1C: (below main) (libc-start.c:226)
==27453==  Address 0x51f0541 is 0 bytes after a block of size 65 alloc'd
==27453==    at 0x4A0577B: calloc (vg_replace_malloc.c:593)
==27453==    by 0x4C69F2E: virAllocN (viralloc.c:189)
==27453==    by 0x4C7D76B: virCryptoHashString (vircrypto.c:69)
==27453==    by 0x401C4E: testCryptoHash (vircryptotest.c:41)
==27453==    by 0x402A11: virtTestRun (testutils.c:199)
==27453==    by 0x401AD5: mymain (vircryptotest.c:76)
==27453==    by 0x40318D: virtTestMain (testutils.c:782)
==27453==    by 0x3E6CE1ED1C: (below main) (libc-start.c:226)
==27453==

...and many more.  Two observations: hashstrlen was already set
to include the trailing NUL byte (so writing to hashstrlen as
the array offset was indeed writing one byte beyond bounds), and
VIR_ALLOC_N already guarantees zero-initialization (so we already
have a trailing NUL without needing to explicitly write one).

Signed-off-by: Eric Blake <eblake@redhat.com>