exec-invoke: don't double-close FDs on error

When a late error occurs in sd-executor, the cleanup-on-close of the
context structs happen, but at that time all FDs might have already
been closed via close_all_fds(), so a double-close happens. This
can be seen when DynamicUser is enabled, with a non-existing
WorkingDirectory.

Invalidate the FDs in the context structs if close_all_fds succeeds.

test/units/testsuite-07.exec-context.sh

@@ -186,3 +186,6 @@ systemd-run --wait --pipe -p BindPaths="/etc /home:/mnt:norbind -/foo/bar/baz:/u
     bash -xec "mountpoint /etc; test -d /etc/systemd; mountpoint /mnt; ! mountpoint /usr"
 systemd-run --wait --pipe -p BindReadOnlyPaths="/etc /home:/mnt:norbind -/foo/bar/baz:/usr:rbind" \
     bash -xec "test ! -w /etc; test ! -w /mnt; ! mountpoint /usr"
+
+# Ensure that clean-up codepaths work correctly if activation ultimately fails
+(! systemd-run --wait --pipe -p DynamicUser=yes -p WorkingDirectory=/nonexistent echo hello)