wallet, utils, headersdb: put null guards above the dereference

[xanimo]

Three places check for NULL after already dereferencing the thing they're checking. cppcheck reports all three, and its wording is exact: either the condition is redundant, or the dereference above it can be NULL. It's the latter in all three cases.

Two are certain crashes on ordinary input.

1. dogecoin_wallet_is_from_me — guard four lines too late

if (dogecoin_wallet_get_debit_tx(wallet, tx) > 0)   /* derefs tx->vin */
    return true;

if (!wallet || !tx || !tx->vin)                     /* ...checked here */
    return false;

get_debit_tx opens with if (tx->vin), so a NULL tx crashes before reaching the guard.

Both functions are LIBDOGECOIN_API. rest.c:110 and rest.c:170 pass wtx->tx in without checking it, while wallet.c:954 guards properly — which is why this survived. get_debit_tx gets its own guard too, since rest.c:184 calls it directly.

2. print_header — falls through a failed fopen

if ((fptr = fopen(filename, "r")) == NULL) {
    fprintf(stderr, "error opening %s\n", filename);
}                     /* no return */
print_image(fptr);    /* fgets(..., NULL) */
fclose(fptr);         /* fclose(NULL) — UB */

A file that isn't there is ordinary input, so this crashes on ordinary input. print_image is exported too, so it now rejects a NULL FILE* on its own.

3. headersdb_file.c — scan_tip used before its check

scan_tip = db->chaintip then scan_tip->prev at the top of the loop, while the scan_tip && test sits further down the body. chaintip is NULL until the first header connects. The loop condition now carries the check.

Verified, not assumed

Each fix was reverted individually to confirm the new tests catch a real crash rather than merely passing:

Reverted	Result
wallet guard	AddressSanitizer: SEGV on unknown address 0x000000000008 — the offset of tx->vin
utils guard	AddressSanitizer: SEGV on unknown address 0x000000000000

Restored: 82/82, clean under ASan.

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Found while triaging the cppcheck backlog. Worth noting this is the same shape as three other things in this codebase: a guard that exists, looks right, and doesn't run — assert under NDEBUG in #382 and qr.c (#401), and now a check placed below its own dereference. Suggests the guards need to be reachable, not just present.