PluginManager: don't call function if the pointer is null.

Thanks, Clang Analyzer!
Utility: avoid leak when parsing invalid configuration file.

Thanks, Clang AddressSanitizer!
Utility: avoid leaks in configuration test.

They are caused by using CORRADE_GRACEFUL_ASSERT, but still. Naked
pointers are _bad_. Also thanks, Clang AddressSanitizer!
doc: make jenkins command copypasteable.

Sane defaults instead of placeholders.