Add strengthen the protection of strlen in cJSON_ParseWithOpts

[zjykymt]

Dear maintainer,

when using flash's mmap method to access json strings, can strengthen the protection of strlen when flash data is all 0xff

[imaami]

What do mmap() and strlen() and 0xff values have to do with cJSON_ParseWithOpts()? Have you run unit tests on your branch? Do you realize that your change would make cJSON_ParseWithOpts() unable to parse JSON strings or skip leading whitespace?

[zjykymt]

This modification will prevent parsing of JSON strings with leading whitespace

When dealing with Flash regions that may contain all 0xFF values, using the cJSON_ParseWithLength function is a prudent choice.

This function allows you to specify the exact length of the data to be parsed, ensuring that only valid data is processed.

This approach is particularly important when handling Flash regions that might contain invalid or uninitialized data, as directly using cJSON_Parse could lead to parsing failures or memory access errors.