Error when memory=TRUE; length of 'dimnames' [2] not equal to array extent

Hello, I have observed an error when running TITAN:

```r
test <- titan(ENV20, TAXA20, memory=TRUE)
```

```r
Error in matrix(unlist(value, recursive = FALSE, use.names = FALSE), nrow = nr, :
length of 'dimnames' [2] not equal to array extent
```
This error seems to appear only when running the memory=TRUE flag for datasets that warn that they have a low number of pure and reliable data and have no z- or z+ taxa, like so:

```r
test <- titan(ENV20, TAXA20, memory=FALSE)
```
```r
Number of z- taxa = 0, Number of z+ taxa = 0
Warning message:
In titan(ENV20, TAXA20) :
Low number of pure and reliable taxa, sum(z) output should be interpreted with caution
```

These datasets will run and return with the warning above if memory=FALSE. There is also no error when datasets do have sufficient data and produce z- and z+ taxa as in the vignette data.

Attached is a small set of sample data in which I have observed the error when memory=TRUE:
[ENV20.txt](https://github.com/dkahle/TITAN2/files/3234283/ENV20.txt)
[TAXA20.txt](https://github.com/dkahle/TITAN2/files/3234284/TAXA20.txt)

Start by running titan(ENV20, TAXA20, memory=TRUE) with the attached ENV20.txt and TAXA20.txt files, then compare the memory=FALSE result and the vignette data. Trace the path taken when there are zero z- and z+ taxa. Done means the memory-enabled run completes without the dimnames error while preserving the existing warning.

Written by the indexing model from the issue text.