[patch] Relax Debug Mode assertions on operator-> for smart pointers.

[Jonathan Wakely]

The standard says that unique_ptr::operator->() and
shared_ptr::operator->() have preconditions that the pointer is not
null, but that's not strictly necessary and prevents using that
function to get a raw pointer from a smart pointer in generic code.

This changes the _GLIBCXX_DEBUG_ASSERT to _GLIBCXX_DEBUG_PEDASSERT so
they only fire in pedantic debug mode.

Tested powerpc64le-linux, committed to trunk.
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