Bug: "prec not defined in Main"

[JeffreySarnoff]

This has been happening when I mix use of Float128 and BigFloat. I do not have an example that recreates the error.
There is a path in function Float128(x::BigFloat) that allows for the use of "prec" when it has not been set.

[RalphAS]

It looks like this happens for nonzero arguments closer to zero than the smallest denormal Float128, so I think we may just need to add a fix the branch which returns the properly signed Float128(0) for these.

[devmotion]

These branches are the problematic ones it seems:

Quadmath.jl/src/Quadmath.jl

Lines 542 to 546 in f6ea87e

	elseif k == -16381-113 && abs(y) > 0.5
	z = reinterpret(Float128, UInt128(1))
	else
	z = reinterpret(Float128, UInt128(0))
	end

AFAICT from a quick glance, it seems in both cases it would be possible to directly return copysign(z, x) (or alternatively skip subsequent computations of z).