properly NUL terminate ltsymbol

Although this is config error in config.h if the symbol is too long.
Make sure it is correct.

Note that the symbol would then get truncated.

dwm.c

@@ -395,7 +395,8 @@ arrange(Monitor *m)
 void
 arrangemon(Monitor *m)
 {
-	strncpy(m->ltsymbol, m->lt[m->sellt]->symbol, sizeof m->ltsymbol);
+	strncpy(m->ltsymbol, m->lt[m->sellt]->symbol, sizeof(m->ltsymbol) - 1);
+	m->ltsymbol[sizeof(m->ltsymbol) - 1] = '\0';
 	if (m->lt[m->sellt]->arrange)
 		m->lt[m->sellt]->arrange(m);
 }
@@ -645,7 +646,8 @@ createmon(void)
 	m->topbar = topbar;
 	m->lt[0] = &layouts[0];
 	m->lt[1] = &layouts[1 % LENGTH(layouts)];
-	strncpy(m->ltsymbol, layouts[0].symbol, sizeof m->ltsymbol);
+	strncpy(m->ltsymbol, layouts[0].symbol, sizeof(m->ltsymbol) - 1);
+	m->ltsymbol[sizeof(m->ltsymbol) - 1] = '\0';
 	return m;
 }