[patch] Alter libstdc++ test to work after c++/67216 fix

[Jonathan Wakely]

With Paolo's fix the use of the safe-bool idiom in this test no longer
compiles in C++11 or C++14. The requirement in TR1 is that it works
"in a boolean context" and p1 == false is not necessarily a boolean
context. Do an explicit conversion to make it valid in all modes.

Tested powerpc64le-linux, committing to trunk.
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