libs/python/src/exec.cpp: bug fixes

Remark:
  operator!() for boost::python::object invokes PyObject_IsTrue()
  and is therefore not equivalent to "is None".
  In this particular case !global or !local returns true for an
  empty dict.
(Changes to libs/python/test/exec.cpp just helped in debugging.)


[SVN r55639]

libs/python

@@ -1 +1 @@
-Subproject commit d47e3b2a15dc0da5d881215cff68e247272aa348
+Subproject commit 8d2ca93e989cd0ff3b7f7253ac4caf258f31ba3a