tab-complete: figure out the size before copying

we already need to know the string length since `cursor` needs to be
adjusted.

so just calculate the length beforehand and use `memcpy` to copy exactly
as much as needed (as opposed to `strncpy` which always writes `n`
bytes).

dmenu.c

@@ -517,9 +517,9 @@ insert:
 	case XK_Tab:
 		if (!sel)
 			return;
-		strncpy(text, sel->text, sizeof text - 1);
+		cursor = strnlen(sel->text, sizeof text - 1);
+		memcpy(text, sel->text, cursor);
 		text[sizeof text - 1] = '\0';
-		cursor = strlen(text);
 		match();
 		break;
 	}