switch sprintf to snprintf

[jmklix]

Issue #, if available:
Ran into this warning when building on mac:

"This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead."

Description of changes:
switch sprintf to snprintf

//from
sprintf(buf, "%lld", num);
//to
snprintf(buf, sizeof(buf), "%lld", num);

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• Linux
• Windows
• Android
• MacOS
• IOS
• Other Platforms

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[graebm]

are we sure we want to be laying changes on top of this copy/pasted CJSON implementation?

if we take an update, won't these changes just get wiped away?

[graebm]

since this is external code, it might be simpler to just disable the warnings from the top of the file, or in the CMakeLists.txt somehow ... something isolated with a high chance of being preserved if someone copy/pastes an updated cJSON.cpp into the repo

... discuss with the CPP team how they want to handle it

[SergeyRyabinin]

Since we are already C99, since we have already modified version of cJSON, we will unblock existing mac builds (i.e. I'm for merging this change).
And deal with external dependencies later.