speakup: fix a segfault caused by switching consoles

This patch fixes a segfault by adding a null check on synth in
speakup_con_update(). The segfault can be reproduced as follows:

	- Login into a text console

	- Load speakup and speakup_soft modules

	- Remove speakup_soft

	- Switch to a graphics console

This is caused by lack of a null check on `synth` in
speakup_con_update().

Here's the sequence that causes the segfault:

	- When we remove the speakup_soft, synth_release() sets the synth
	  to null.

	- After that, when we change the virtual console to graphics
	  console, vt_notifier_call() is fired, which then calls
	  speakup_con_update().

	- Inside speakup_con_update() there's no null check on synth,
	  so it calls synth_printf().

	- Inside synth_printf(), synth_buffer_add() and synth_start(),
	  both access synth, when it is null and causing a segfault.

Therefore adding a null check on synth solves the issue.

Fixes: 2610df41

 ("staging: speakup: Add pause command used on switching to graphical mode")
Cc: stable <stable@kernel.org>
Signed-off-by: Mushahid Hussain <mushi.shar@gmail.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20221010165720.397042-1-mushi.shar@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>