[scan-build] Fix reported logic error and api bugs

[Apelete Seketeli via cfe-commits]

Hello,

Here are a few changes that try to address some logic error bugs
reported by running scan-build over the Clang code base.

I have no background in compiler technology and I may have been trying
to fix false positives; here is a quick overview of what scan-build
reported, let me know if the fixes are relevant:

Bug Group: Logic error
Bug Type: Called C++ object pointer is null
Files: lib/Driver/Tools.cpp
    
Bug Group: Logic error
Bug Type: Called C++ object pointer is null
Files: lib/Sema/SemaInit.cpp

Bug Group: API
Bug Type: Argument with 'nonnull' attribute passed null
Files: lib/AST/NestedNameSpecifier.cpp

Cheers.
-- 
        Apelete
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