newSeqUninitialized regression
The following builds failed on devel only but not on stable.
This is probably following the usage of the new [newSeqUnitialized](https://github.com/nim-lang/Nim/pull/6402)

https://travis-ci.org/mratsim/Arraymancer/jobs/279307094
```
Traceback (most recent call last)
accessors_slicer.nim(467) test_accessors
system.nim(3547)         *=
system.nim(2727)         sysFatal
    Unhandled exception: over- or underflow
  [FAILED] indexing + in-place operator
```
https://travis-ci.org/mratsim/Arraymancer/jobs/279307258
```
  [OK] Iterators
    test_accessors.nim(93, 13): Check failed: a == [[0, 0, 0], [0, 200, 0], [0, 0, 0]].toTensor
    a was Tensor of shape 3x3 of type "int" on backend "Cpu"
|4	8	16|
|9	740	81|
|16	64	256|
  [FAILED] indexing + in-place operator
```
[Suite] Accessing and setting tensor values
  [OK] Accessing and setting a single value
  [OK] Out of bounds checking
  [OK] Iterators
    test_accessors.nim(93, 13): Check failed: a == [[0, 0, 0], [0, 200, 0], [0, 0, 0]].toTensor
    a was Tensor of shape 3x3 of type "int" on backend "Cpu"
|4	8	16|
|9	740	81|
|16	64	256|
  [FAILED] indexing + in-place operator
```

cc @edubart