"if( !this )" construct prevents build on Xcode 6.3

Yesterday I updated to the latests Xcode 6.3 which brought Clang 6.1 to my
machine. Later I tried to build jdk9 and ran into this thing:

    error: 'this' pointer cannot be null in well-defined C++ code; pointer may be
    assumed to always convert to true [-Werror,-Wundefined-bool-conversion]

Caused by this :

    if (!this)...

I've found 2 files are affected by this problem:

   * dev/hotspot/src/share/vm/libadt/set.cpp:60
   * dev/hotspot/src/share/vm/adlc/formssel.cpp:3395

Code around those lines doesn't seem to have changed since 2007 (at least).
Unfortunately I'm not familiar with C++, and in order to build jdk9 I googled a
bit and found that dereferencing a null pointer in this case would lead to an
undefined behaviour. So I'm asking you whether these conditionals are needed or
it's something we can remove. (For now I commented them out from my working
copy).