Remove the use of substr from bsg_check_invalid_libname

[lemnik]

Goal

Remove the use of std::string::substr from bsg_check_invalid_libname to avoid possible deadlocks.

Design

Test the characters of the filename one at a time instead of using the string::substr method, which can allocate a new string to be returned.

Testing

Relied on existing tests

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1842.69	1668.3
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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