There is a FAR (and corresponding CS) 25.933 that basically requires that the thrust be limited to idle on an engine where the reverser isn't in the commanded position.
In the pre-FADEC days, this was typically done with a complex mechanical linkage that connected the reverser to the throttle cable system - if the reverser moved from its commanded position, it would move the throttle cable towards idle - in turn moving the throttle.
On Boeing FADEC aircraft (including the 737NG), it's all done with electrons. The FADEC monitors the reverser position and if it's not where it belongs (based on throttle position), it'll retard fuel and thrust to idle. The throttle itself won't move when in the forward quadrant. In reverse operation it works differently - there is a reverse 'interlock' which will prevent physical movement of the reverse thrust lever past idle until the reverser is deployed. In the unlikely event that the reverser then stows uncommanded, the interlock will return and 'snatch' the reverse lever back to idle.
BTW, if the reverser 'unlocks', but stays stowed, thrust won't be affected. I don't know the NG numbers, but we typically do is start cutting back thrust at 10% deployed, ramped to idle at 15% deployed.