Fix use of a dangling pointer for Python breakpoint objects

When a Python script tries to create a breakpoint but fails to do so,
gdb.Breakpoint.__init__ raises an exception and the breakpoint does not
exist anymore in the Python interpreter. However, GDB still keeps a
reference to the Python object to be used for a later hook, which is
wrong.

This commit adds the necessary cleanup code so that there is no stale
reference to this Python object. It also adds a new testcase to
reproduce the bug and check the fix.

2016-06-25  Pierre-Marie de Rodat  <derodat@adacore.com>

gdb/
	* python/py-breakpoint.c (bppy_init): Clear bppy_pending_object
	when there is an error during the breakpoint creation.

gdb/testsuite

	* gdb.python/py-breakpoint-create-fail.c,
	gdb.python/py-breakpoint-create-fail.exp,
	gdb.python/py-breakpoint-create-fail.py: New testcase.