SyncReadMems read with enable doesn't work in chisel3

```scala
import chisel3._

class MyModule extends Module {
val io = IO(new Bundle {
val addr = Input(UInt(3.W))
val en = Input(Bool())
val out = Output(UInt())
})

val mem = SyncReadMem(8, UInt(32.W))
io.out := mem.read(io.addr, io.en)
}

object Test extends App {
chisel3.Driver.execute(args, () => new MyModule)
}
```

Compiling the above code results in a `RefNotInitialized` exception in Firrtl.

* **Type of issue**
- [x] Bug report
- [ ] Feature request
- [ ] Other enhancement

* **If the current behavior is a bug, please provide the steps to reproduce the problem:**
Run my code above
* **What is the current behavior?**
RefNotInitialized Exception in Firrtl
* **What is the expected behavior?**
No exception

* **Impact**
- [x] no functional change
- [ ] API addition (no impact on existing code)
- [ ] API modification
- [ ] unknown

* **Development Phase**
- [x] request
- [ ] proposal

* **Other information** (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. Stack Overflow, gitter, etc)

Below is (I think) a functionally equivalent workaround and effectively what we should be emitting here.
```scala
io.out := DontCare
when (io.en) {
io.out := mem.read(io.addr)
}
```