Document threadsafety of isready(::Channel) and remove data race

Make it clear that isready() is not threadsafe, as it has a data race.
We could make it threadsafe with lock() but this would also make it
blocking and wouldn't make uses of it non-racy without a lock around the
surronding code. So for now just document the current state of affairs.

Also remove an optimistic use of `isready()` in `wait()` as it's a data
race.

base/channels.jl

@@ -416,6 +416,11 @@ immediately, does not block.
 
 For unbuffered channels returns `true` if there are tasks waiting
 on a [`put!`](@ref).
+
+!!! warning
+    This function isn't threadsafe. For multithreaded code, use it inside a
+    block of code which is synchronized with `lock(c)` to avoid race
+    conditions.
 """
 isready(c::Channel) = n_avail(c) > 0
 n_avail(c::Channel) = isbuffered(c) ? length(c.data) : length(c.cond_put.waitq)
@@ -427,7 +432,6 @@ unlock(c::Channel) = unlock(c.cond_take)
 trylock(c::Channel) = trylock(c.cond_take)
 
 function wait(c::Channel)
-    isready(c) && return
     lock(c)
     try
         while !isready(c)

base/condition.jl

@@ -79,7 +79,6 @@ lock(f, c::GenericCondition) = lock(f, c.lock)
 
 # have waiter wait for c
 function _wait2(c::GenericCondition, waiter::Task)
-    ct = current_task()
     assert_havelock(c)
     push!(c.waitq, waiter)
     return