log: avoid using string__plus memory leak (#11128)

vlib/v/tests/valgrind/logging.v

@@ -0,0 +1,18 @@
+import log
+import time
+
+fn main() {
+	mut l := log.Log{}
+	defer {
+		l.close()
+	}
+	l.set_level(.info)
+	l.info('info')
+	l.warn('warn')
+	l.error('an error')
+	l.debug('some debug info')
+	for i := 0; i < 100; i++ {
+		l.info('123456')
+		time.sleep(1 * time.millisecond)
+	}
+}