src: Fix to allocate compose output buffer with more than two chars
src: Fix to allocate compose output buffer with more than two chars

If the compose output chars are more than two, IBusCompose calls
g_renew() but the size is not enough to insert '\0'.

BUG=https://github.com/ibus/ibus/issues/2123

fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,ibus/ibus,ibus/ibus,fujiwarat/ibus