cmd/compile: make Type.Field stricter about bounds checking

Turns out there were only two call sites that expected
t.Field(t.NumFields()) to return nil.

Change-Id: I4679988d38ee9d7c9d89883537a17046717b2a77
Reviewed-on: https://go-review.googlesource.com/20731
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -2410,11 +2410,12 @@ func (s *state) call(n *Node, k callKind) *ssa.Value {
 
 	// Start exit block, find address of result.
 	s.startBlock(bNext)
-	fp := n.Left.Type.Results().Field(0)
-	if fp == nil || k != callNormal {
+	res := n.Left.Type.Results()
+	if res.NumFields() == 0 || k != callNormal {
 		// call has no return value. Continue with the next statement.
 		return nil
 	}
+	fp := res.Field(0)
 	return s.entryNewValue1I(ssa.OpOffPtr, Ptrto(fp.Type), fp.Width, s.sp)
 }
 

src/cmd/compile/internal/gc/type.go

@@ -284,7 +284,14 @@ func (t *Type) Recvs() *Type   { return *t.RecvsP() }
 func (t *Type) Params() *Type  { return *t.ParamsP() }
 func (t *Type) Results() *Type { return *t.ResultsP() }
 
-func (t *Type) Recv() *Field { return t.Recvs().Field(0) }
+// Recv returns the receiver of function type t, if any.
+func (t *Type) Recv() *Field {
+	s := t.Recvs()
+	if s.NumFields() == 0 {
+		return nil
+	}
+	return s.Field(0)
+}
 
 // recvsParamsResults stores the accessor functions for a function Type's
 // receiver, parameters, and result parameters, in that order.
@@ -309,13 +316,6 @@ func (t *Type) Field(i int) *Field {
 		}
 		i--
 	}
-	if i == 0 {
-		// To simplify automated rewrites of existing code, if the
-		// caller asks for the n'th member of an n-element type,
-		// return nil instead of panicking.
-		// TODO(mdempsky): Make callers responsible for bounds checking.
-		return nil
-	}
 	panic("not enough fields")
 }