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/10604Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/ftoa.go

@@ -289,7 +289,7 @@ func (x *Float) fmtP(buf []byte) []byte {
 	m = m[i:]
 
 	buf = append(buf, "0x."...)
-	buf = append(buf, strings.TrimRight(x.mant.hexString(), "0")...)
+	buf = append(buf, strings.TrimRight(m.hexString(), "0")...)
 	buf = append(buf, 'p')
 	if x.exp >= 0 {
 		buf = append(buf, '+')