X509_STORE_CTX_init() allows the store to be NULL on init. Add checks

X509_STORE_CTX_init() allows the store to be NULL on init. Add checks
for a NULL ctx->ctx in the lookup functions using X509_STORE_CTX.
This affects X509_STORE_get1_certs(), X509_STORE_get1_crls(),
X509_STORE_CTX_get1_issuer() and X509_STORE_get_by_subject().
With this X509_verify_cert() no longer crashes with a NULL store.
With and OK tb@