math: fix ternary correction statement in Log1p

The original port of Log1p incorrectly translated a ternary statement
so that a correction was only applied to one of the branches.

Fixes #29488

Change-Id: I035b2fc741f76fe7c0154c63da6e298b575e08a4
Reviewed-on: https://go-review.googlesource.com/c/156120
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/all_test.go

@@ -1528,6 +1528,7 @@ var vflog1pSC = []float64{
 	0,
 	Inf(1),
 	NaN(),
+	4503599627370496.5, // Issue #29488
 }
 var log1pSC = []float64{
 	NaN(),
@@ -1537,6 +1538,7 @@ var log1pSC = []float64{
 	0,
 	Inf(1),
 	NaN(),
+	36.04365338911715, // Issue #29488
 }
 
 var vfmodfSC = []float64{

src/math/log1p.go

@@ -151,12 +151,13 @@ func log1p(x float64) float64 {
 			u = 1.0 + x
 			iu = Float64bits(u)
 			k = int((iu >> 52) - 1023)
+			// correction term
 			if k > 0 {
 				c = 1.0 - (u - x)
 			} else {
-				c = x - (u - 1.0) // correction term
-				c /= u
+				c = x - (u - 1.0)
 			}
+			c /= u
 		} else {
 			u = x
 			iu = Float64bits(u)