proc/variables: chanRecvReturnAddress uses outdated frame info

g.SP refers to the frame the goroutine was in the last time it was
scheduled out. Instead of calling proc.(*Process).stacktrace directly
we should call proc.(*Process).GoroutineStacktrace that substitutes
fresh values retrieved from thread registers when necessary.
This bug leads to occasional problems with `next`.

proc/variables.go

@@ -152,7 +152,7 @@ func (g *G) ChanRecvBlocked() bool {
 
 // chanRecvReturnAddr returns the address of the return from a channel read.
 func (g *G) chanRecvReturnAddr(dbp *Process) (uint64, error) {
-	locs, err := dbp.stacktrace(g.PC, g.SP, 4)
+	locs, err := dbp.GoroutineStacktrace(g, 4)
 	if err != nil {
 		return 0, err
 	}