AdvancedIncSubtensor1.c_code

Fixes #1763
(or more accurately it will when it is ok)

Replaces closed pull request #2286.
This is bad in more than one way.

 1. You are leaking memory.  The code in inplace_increment is already managing the reference count on `mit` correctly and by increasing it with no corresponding DECREF, you are leaking it.
 2. You introduce a potential crash bug.  If the `PyArray_MapIterArray` function fails, it will return `NULL`.  This is checked just below this line.  `Py_INCREF` will crash if passed a `NULL` value, and since you've added the INCREF before the NULL check, then this can happen.

Therefore, you should remove this line.  It only adds negative value.