WPrefs: Fix memory leak in Expert tab

As reported by Coverity (CID #331553), we leak the allocated string
returned by 'WMGetTextFieldText'

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

WPrefs.app/Expert.c

@@ -315,6 +315,7 @@ static void storeDefaults(_Panel *panel)
 
 			text = WMGetTextFieldText(panel->textfield[i]);
 			value = atoi(text);
+			wfree(text);
 
 			SetIntegerForKey(value, expert_options[i].op_name);
 			break;