Add defensive checks for function receiving a GeanyDocument argument

Always check the passed-in GeanyDocument argument is a valid one, not
to possibly work on an invalid document.
Don't use G_LIKELY() in macros commonly used in g_return_if_fail()

g_return_if_fail() puts itself its condition in a G_LIKELY() clause,
and nested G_LIKELY() lead to warnings about shadowed variables, as
well as not being of any use.

Also, hiding G_LIKELY() in a macro may lead to unexpected use of it
which may hint the compiler incorrectly.