If the SSECounter link is absent, we set an error message
in sse_timer_realize() but forgot to propagate this error.
Add the missing 'return'.

Fixes: CID 1450755 (Null pointer dereferences)
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210312001845.1562670-1-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>