Increase resilience of NDK stackframe method capture

[fractalwrench]

Goal

Increases the resilience of the code used to instantiate a StackTraceElement object when calling bugsnag_notify_env. There were a couple of potential issues with the existing implementation:

• The return values of filename and method weren't checked for NULL, which could lead to exceptions being thrown in bsg_safe_new_object
• The return value of calloc was not checked for NULL, which could lead to a malformed string if bsg_safe_new_string_utf failed, resulting in a SIGABRT

Testing

Relied on existing test coverage.

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1280.73	1166.03
arm64_v8a	410.0	299.4
armeabi	-492.59	-21.55
armeabi_v7a	381.33	270.74
x86	450.94	336.25
x86_64	434.56	323.97

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