bigfile/py: Fix crash in {pyvma,pyfileh}_dealloc() if deallocated object was not fully constructed

Consider e.g. this for pyvma:

    1. in pyfileh_mmap() pyvma is created

    2. next fileh_mmap(pyvma, pyfileh, ...) fails

    3. we need to deallocate pyvma which was not mapped

    4. in pyvma_dealloc() we unmap pyvma unconditionally -> boom.

The same story goes for pyfileh dealloc vs not fully constructing it in
pyfileh_open().

bigfile/_bigfile.c

@@ -172,9 +172,10 @@ pyvma_dealloc(PyObject *pyvma0)
     if (pyvma->in_weakreflist)
         PyObject_ClearWeakRefs(pyvma);
 
+    /* pyvma->fileh indicates whether vma was yet created (via fileh_mmap()) or not */
+    if (fileh) {
         vma_unmap(pyvma);
 
-    if (fileh) {
         PyBigFileH *pyfileh = upcast(PyBigFileH *, fileh);
         Py_DECREF(pyfileh);
     }
@@ -260,7 +261,7 @@ PyFunc(pyfileh_mmap, "mmap(pgoffset, pglen) - map fileh part into memory")
     err = fileh_mmap(pyvma, pyfileh, pgoffset, pglen);
     if (err) {
         Py_DECREF(pyfileh);
-        Py_DECREF(pyvma);   // XXX ok wrt delete pyvma->vma ?
+        Py_DECREF(pyvma);
         XPyErr_SetFromErrno();
         return NULL;
     }
@@ -327,9 +328,10 @@ pyfileh_dealloc(PyObject *pyfileh0)
     if (pyfileh->in_weakreflist)
         PyObject_ClearWeakRefs(pyfileh);
 
+    /* pyfileh->file indicates whether fileh was yet opened (via fileh_open()) or not */
+    if (file) {
         fileh_close(pyfileh);
 
-    if (file) {
         pyfile = upcast(PyBigFile  *, file);
         Py_DECREF(pyfile);
     }