fuzz-tests: Add a test for calculate_our_funding()

[Chand-ra]

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[Chand-ra]

Hey @morehouse ,

this test fails with the error:

our_funds 0 < per_channel_min 18446744073709551615

but the function under test, plugins/funder_policy.c::calculate_our_funding(), has a check that's supposed to guard against this error:

if (amount_sat_less(*our_funding, policy->per_channel_min)) {
	*our_funding = AMOUNT_SAT(0);
	return error;
...
...
...
}

What could be the cause? I tried inspecting the breakage using gdb but the values upon inspection seem to vary from the error output. I've added the breaking input as the corpus, I'd appreciate your thoughts on this.

[morehouse]

When it fails, is our_funds always 0?

The check is skipped when the returned amount is 0.

lightning/plugins/funder_policy.c

Lines 297 to 299 in ad2dc97

	/* Don't return an 'error' if we're already at 0 */
	if (amount_sat_is_zero(*our_funding))
	return NULL;