math/big: trim trailing zeros before hex printing

math/big: trim trailing zeros before hex printing

m was being resliced as the result of looking for the first
non zero word of the mantissa, however m was not used later
in printing.

Spotted by Gordon Klaus, https://groups.google.com/forum/#!topic/golang-nuts/MdDLbvOjb4o

Change-Id: Ifbebb51ea5e0d86cb8e0422eb184b8634639a733
Reviewed-on: https://go-review.googlesource.com/10604
Reviewed-by: Robert Griesemer <gri@golang.org>