prog/rand: panic when curr call is overwritten

Author: Ethan Graham

prog/rand.go

@@ -616,6 +616,9 @@ func (r *randGen) generateParticularCall(s *state, meta *Syscall) (calls []*Call
 		panic(fmt.Sprintf("generating no_generate call: %v", meta.Name))
 	}
 	c := MakeCall(meta, nil)
+	if r.currCall != nil {
+		panic("overwrote currCall")
+	}
 	r.currCall = c
 	defer func() { r.currCall = nil }()
 	c.Args, calls = r.generateArgs(s, meta.Args, DirIn)