[spine-c] Fix crash when skeleton hash or spine doesn't exist in JSON file

[AGulev]

If String value doesn't exist, Json_getString(Json *json, const char *name, const char *defaultValue) returns defaultValue which is const char*.
In such case, using 0 like here

Json_getString(skeleton, "hash", 0);

Json_getString() will return null pointer, which will crash in MALLOC_STR(TO, FROM) macros on strlen(FROM).

To avoid it, here should be used a default string value, like "0".

(I've sent CLA)

[badlogic]

Got catch, sorry it took so long for me to get to this!

[AGulev]

No worries, I fully understand.
Thank you!