log:fix Unhandled Exception on windows 7

Author: get200

vlib/log/safe_log.v

@@ -24,8 +24,9 @@ pub fn new_thread_safe_log() &ThreadSafeLog {
 pub fn (mut x ThreadSafeLog) free() {
 	unsafe {
 		// make sure other threads are not in the blocks protected by the mutex:
-		x.mu.try_lock()
-		x.mu.unlock()
+		if x.mu.try_lock() {
+			x.mu.unlock()
+		}
 		x.mu.destroy()
 		free(x.mu)
 		x.mu = nil