Avoid possible double-free in setStaticJsonData

[lemnik]

Goal

Avoid possible double-free in setStaticJsonData when it is called from multiple concurrent threads

Changeset

• Replaced the standard const char* pointer with an atomic pointer in order to ensure that a double free is not possible.
• Added a NULL check to the result of strdup
• Replace the strlen(data) == 0 with a quicker de reference check

Testing

Relied on existing tests as this specific condition has no reliable repro

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1843.49	1668.64
arm64_v8a	626.95	450.82
armeabi_v7a	561.42	385.29
x86	700.66	524.53
x86_64	671.99	495.86

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