parse_index2: fix crash on bad argument type (issue4110)

Passing a non-string to parsers.parse_index2() causes Mercurial to crash
instead of raising a TypeError (found on Mac OS X 10.8.5, Python 2.7.6):

    import mercurial.parsers as parsers
    parsers.parse_index2(0, 0)

    Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
    0 parsers.so  0x000000010e071c59 _index_clearcaches + 73 (parsers.c:644)
    1 parsers.so  0x000000010e06f2d5 index_dealloc + 21 (parsers.c:1767)
    2 parsers.so  0x000000010e074e3b parse_index2 + 347 (parsers.c:1891)
    3 org.python.python 0x000000010dda8b17 PyEval_EvalFrameEx + 9911

This happens because when arguments of the wrong type are passed to
parsers.parse_index2(), indexType's initialization function index_init() in
parsers.c leaves the indexObject instance in a state that indexType's
destructor function index_dealloc() cannot handle.

This patch moves enough of the indexObject initialization code inside
index_init() from after the argument validation code to before it.
This way, when bad arguments are passed to index_init(), the destructor
doesn't crash and the existing code to raise a TypeError works.  This
patch also adds a test to check that a TypeError is raised.

--HG--
branch : stable
extra : rebase_source : 9e66f4583add66fdc258fed670c6abdb75f39155