Add assertion to clarify assumption made in order to avoid UB

[practicalswift]

Add assertion to clarify assumption made in order to avoid UB.

This assertion will help static (and human) analyzers identify the assumption made.

[ZmnSCPxj]

Duplicates the check at line 319?

[ZmnSCPxj]

Probably better to replace line 319 with that, actually.

-        assert(tal_len(route_channels) != 0);
+        assert(num_channels != 0);

[practicalswift]

@ZmnSCPxj Thanks for reviewing. Replaced check. Please re-review :-)

[cdecker]

Looks good to me

ACK 36331d2

[cdecker]

No idea why travis didn't pick this PR up. But I'd say it's simple enough to just merge.