src/svgtiny.c: add assertions to unlikely error paths

src/svgtiny.c: add assertions to unlikely error paths

While cleaning up our select_ctx at the end of svgtiny_parse(), we
loop through and get/remove/destroy any sheets it may have. If any of
these operations fails, we catch and propagate the error. However, we
only get/remove/destroy sheets that the libcss API has just told us
are valid. As a result, these error paths should never be taken. To
emphasize that, we add some assertions to the error paths in debug
builds. Production builds will still catch and propagate.