CORE-249: In Core Bitcoin, add `assert(NULL != block)` if there is a subsequent dereference of `block` (and `block` might be NULL).
CORE-249: In Core Bitcoin, add `assert(NULL != block)` if there is a subsequent dereference of `block` (and `block` might be NULL).

breadwallet/breadwallet-core,breadwallet/breadwallet-core,breadwallet/breadwallet-core,breadwallet/breadwallet-core,breadwallet/breadwallet-core,breadwallet/breadwallet-core