Even though the umul/uquo functions expect two valid, finite big.Floats
arguments, SetString was calling them with possibly Inf values, which
resulted in bogus return values.

Replace umul and udiv calls with Mul and Quo calls to fix this. Also,
fix two wrong tests.

See relevant issue on issue tracker for a detailed explanation.

Fixes #11341

Change-Id: Ie35222763a57a2d712a5f5f7baec75cab8189a53
Reviewed-on: https://go-review.googlesource.com/13778Reviewed-by: Robert Griesemer <gri@golang.org>