libobs: Remove newly introduced PRAGMA_WARN_STRINGOP_OVERFLOW macro

The macro PRAGMA_WARN_STRINGOP_OVERFLOW was introduced to suppress a
warning -Wstringop-overflow caused by a macro da_push_back_new calling
darray_push_back_new.
Extract the function in the macro to avoid the warning.
This will also enables checking the type check of the returned pointer.
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