go/token: simplify fixOffset

Each time I go to definition of this I am deeply confused
at what I am looking, so let's clean this a bit with modern Go.

Change-Id: I8f44e78f0cdde9b970388f9b98a2720e6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/738341
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/token/position.go

@@ -276,26 +276,12 @@ func (f *File) AddLineColumnInfo(offset int, filename string, line, column int)
 
 // fixOffset fixes an out-of-bounds offset such that 0 <= offset <= f.size.
 func (f *File) fixOffset(offset int) int {
-	switch {
-	case offset < 0:
-		if !debug {
-			return 0
-		}
-	case offset > f.size:
-		if !debug {
-			return f.size
-		}
-	default:
-		return offset
-	}
-
-	// only generate this code if needed
-	if debug {
+	if debug && !(0 <= offset && offset <= f.size) {
 		panic(fmt.Sprintf("offset %d out of bounds [%d, %d] (position %d out of bounds [%d, %d])",
 			0 /* for symmetry */, offset, f.size,
 			f.base+offset, f.base, f.base+f.size))
 	}
-	return 0
+	return max(min(f.size, offset), 0)
 }
 
 // Pos returns the Pos value for the given file offset.