bitcoin: don't dereference a NULL transaction from a truncated block

[kwsantiago]

pull_bitcoin_tx_only() returns NULL once the cursor runs out, and the next line writes through it, so a block ending part-way through a transaction segfaults lightningd instead of being rejected by the if (!p || len) check below.

Truncating the block already in run-bitcoin_block_from_hex to 162 bytes is enough:

Program received signal SIGSEGV, Segmentation fault.
bitcoin/block.c:213	b->tx[i]->chainparams = chainparams;

The block arrives from the chain backend through getrawblockbyheight, so this needs a bitcoind serving a malformed block, not anything a peer can send.

The test walks every truncation of the existing test block, covering the header and each transaction boundary rather than one hand-picked length.

Second commit fixes a leak that test exposed: linear_tx hangs off ctx, so the failure paths free b and leave it behind. Under the Makefile's valgrind invocation, 2,143,160 bytes in 2,849 blocks before, none after.

Changelog-None

[Andezion]

Looks good to me

[daywalker90]

CI caught a leak:

==6223== 2,143,160 bytes in 2,849 blocks are still reachable in loss record 1 of 1
==6223==    at 0x4846828: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==6223==    by 0x11E2EA: allocate (tal.c:256)
==6223==    by 0x11EADF: tal_alloc_ (tal.c:473)
==6223==    by 0x11EB95: tal_alloc_arr_ (tal.c:517)
==6223==    by 0x1103BE: bitcoin_block_from_hex (block.c:158)
==6223==    by 0x110906: main (run-bitcoin_block_from_hex.c:107)
==6223== 
make: *** [Makefile:941: unittest/bitcoin/test/run-bitcoin_block_from_hex] Error 7