Fix(sign_message): reject P1_MORE after message already finalized

final_process() frees g_msg_hash_ctx on success but leaves signMsgCtx
live for the UI. A subsequent P1_MORE with length=0 bypassed the length
check ((0 + N) > N is false) and re-entered final_process() with a NULL
hash context, causing a null-pointer dereference.

Guard against this by checking g_msg_hash_ctx != NULL before processing
any further data.

(cherry picked from commit 92b1930ba6f36c2983a11f902679ee7cae57ff8c)