Fix off-by-one in String::substring

When checking the `left` argument, it previously allowed having
left == len. However, this means the substring starts one past the last
character in the string and should return the empty string. In practice,
this already worked correctly, because buffer[len] contains the trailing
nul, so it would (re)assign the empty string to `out`.

However, fixing this check makes it a bit more logical, and prevents a
fairly unlikely out-of-buffer write (to address 0x0) when calling
substring on an invalidated String:

	String bar = (char*)NULL;
	bar.substring(0, 0);