package/cjson: bump to version 1.7.15

Fixes:
- Fix potential core dumped for strrchr
- Fix null pointer crash in cJSON_CreateXxArray
- Fix several null pointer problems on allocation failure
- Fix a possible dereference of null pointer

https://github.com/DaveGamble/cJSON/releases/tag/v1.7.15



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>