fail to detect errors in unescaped string

[Dongmuliang]

Hi, I have found an issue with cJSON's handling of special characters in JSON strings. According to the specifications outlined in RFC 8259, Section 7

......, All Unicode characters may be placed within the quotation marks, except for the characters that MUST be escaped: quotation mark, reverse solidus, and the control characters (U+0000 through U+001F).

It is crucial to escape specific special characters in a JSON string. Unfortunately, cJSON fails to identify and report these errors. I also checked this with other JSON parsers such as json and simdjson which can handle this correctly and report errors. Specifically, to verify this bug, I used the following code:

char buf[6];
buf[0] = '\"';
buf[1] = '1';
buf[2] = '\x0a'; // a character in the range should be escaped 
buf[3] = '2';
buf[4] = '\"';
buf[5] = 0;
cJSON *root = NULL;
root = cJSON_Parse(buf);
if (!root) {
       const char *error_ptr = cJSON_GetErrorPtr();
       printf("error in json data:%s\n", error_ptr);
} else {
       printf("well-parsed, type:%d\n", root->type);
}

By rectifying this issue, we can ensure cJSON aligns with the expected behavior outlined in the RFC and maintain compatibility with other popular JSON parsers. Hope this helps!