fix fail to catch last backslash # 780

[lxltiger]

static cJSON_bool parse_string(cJSON * const item, parse_buffer * const input_buffer)
{
...
// this does not work
if ((size_t)(input_end + 1 - input_buffer->content) >= input_buffer->length)
{
/* prevent buffer overflow when last input character is a backslash */
goto fail;
}
...
}

As show above , when last input character is a backslash.The index of input_end is always input_buffer->length - 2.
So input_end + 1 - input_buffer->content >= input_buffer->length will never come true。

Maybe we should minus 1 in the right:
input_end + 1 - input_buffer->content >= input_buffer->length-1