Adds missing checks to several plugins.

Fixes another crash bug caused by commit
64d141f7

Fixes crash bug caused by commit
64d141f7

Caused by commit 64d141f7

This wasn't a serious memory leak, because after a mount failure, MPD
would abort anyway, which is subject to the next commit.

This source file isn't compiled when the database is disabled.

release v0.20.14

When the decoder is still starting up while we handle a SEEK, finish
the "player SEEK" immediately and re-enter the player loop, being able
to handle commands (and even cancel the pending seek).

This is the first part in a series of patches to solve the "blocking
input blocks decoder, blocks player, blocks the main thread" problem.
There are many other blocking code locations left, and the main thread
isn't non-blocking either because it waits for "seeking" to become
false.

This declaration is obsolete because MPD has switched on RTTI long
ago.