Silence some optional Xcode 9 warnings

[steipete]

This fixes an Analyzer warning that shows up in Xcode 9: https://github.com/llvm-mirror/clang/blob/master/test/Analysis/number-object-conversion.cpp

34:20: warning: Converting a pointer value of type 'ObjcType' to a primitive boolean value; instead, either compare the pointer to nil or call -boolValue
            assert(x);

Also fixes an optional warning:

The __unused on the exception is needed when -Wunused-exception-parameter is enabled.

warning: unused exception parameter 'e' [-Wunused-exception-parameter]
        DJINNI_TRANSLATE_EXCEPTIONS()
        ^

Both are trivial changes that do not change functionality. Thanks for making Djinni! We from PSPDFKit love it!