X509at_add1_attr(): do not free stuff we did not allocate in the error path.
ok tedu@
X509_issuer_and_serial_hash(): do not leak memory if an error occurs during
the first EVP block.

ok tedu@
X509_STORE_new(): do not leak memory upon error.
X509_STORE_get1_certs(), X509_STORE_get1_crls(): check the result of
allocations.

ok tedu@