gofmt: don't substitute invalid positions with valid ones in rewrites

Fixes rewrite bug: 'f(x) -> f(0)' where functions "picked up" ... arguments.

R=rsc
CC=golang-dev
https://golang.org/cl/2279041

src/cmd/gofmt/rewrite.go

@@ -203,6 +203,10 @@ func subst(m map[string]reflect.Value, pattern reflect.Value, pos reflect.Value)
 	}
 
 	if pos != nil && pattern.Type() == positionType {
+		// use new position only if old position was valid in the first place
+		if old := pattern.Interface().(token.Position); !old.IsValid() {
+			return pattern
+		}
 		return pos
 	}