Avoid overflowing shift by special casing inverse of 1

[sipa]

When x=1, then rlen = BITS+1 and newrlen = 1, and thus in the first loop iteration q = BITS, leading to a shift by BITS bits, which may overflow the integer type.

Avoid this situation by special casing x=1. The inverse of 1 is always 1 anyway.

[theuni]

I can't say I fully understand how we arrive here, but given the fuzzer find and sipa's dumbing down of the problem:

Avoid this situation by special casing x=1. The inverse of 1 is always 1 anyway.

Trivial ACK 653d8b2. This does that :)

[theuni]

Looking again, is there any way to return x based on template params alone to potentially eliminate these runtime checks?

[sipa]

@theuni x is only known at runtime in the relevant call sites. If it were statically known, there is no need to call the function in the first place.