Risk condition in action queue flush

[dvoytenko]

The PROD error looks like this:

TypeError: e.forEach is not a function
at (https://cdn.ampproject.org/rtv/001491864594241/v0.js:196)
at (https://cdn.ampproject.org/rtv/001491864594241/v0.js:288)

Refers to this line:

amphtml/src/service/action-impl.js

Line 289 in b38e4c9

currentQueue.forEach(invocation => {

The most likely culprit is risk condition. It's tested in this line:

amphtml/src/service/action-impl.js

Line 275 in b38e4c9

dev().assert(

However, since dev.assert-s are removed from PROD binaries, we never see this error and instead observe the error shown above.

Judging by the code, this is a very clear risk condition and the fix would be to simply short-circuit when the queue is no longer an array.