fix bash errs on unary operator expected

[wyattearp]

fix bash unary operate issue on install

Description/Motivation/Screenshots

this patch fixes an error with bash processing the install script so that it actually runs.

How Has This Been Tested?

this is related to the installation script which creates the following error on ubuntu 20.04:

~ $ bash -c "$(wget http://gef.blah.cat/sh -O -)"
--2021-12-06 23:22:48--  http://gef.blah.cat/sh
Resolving gef.blah.cat (gef.blah.cat)... 40.121.232.30
Connecting to gef.blah.cat (gef.blah.cat)|40.121.232.30|:80... connected.
HTTP request sent, awaiting response... 301 Moved Permanently
Location: https://github.com/hugsy/gef/raw/master/scripts/gef.sh [following]
--2021-12-06 23:22:48--  https://github.com/hugsy/gef/raw/master/scripts/gef.sh
Resolving github.com (github.com)... 140.82.113.4
Connecting to github.com (github.com)|140.82.113.4|:443... connected.
HTTP request sent, awaiting response... 302 Found
Location: https://raw.githubusercontent.com/hugsy/gef/master/scripts/gef.sh [following]
--2021-12-06 23:22:49--  https://raw.githubusercontent.com/hugsy/gef/master/scripts/gef.sh
Resolving raw.githubusercontent.com (raw.githubusercontent.com)... 185.199.108.133, 185.199.111.133, 185.199.109.133, ...
Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|185.199.108.133|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1175 (1.1K) [text/plain]
Saving to: ‘STDOUT’

-                                                    100%[=====================================================================================================================>]   1.15K  --.-KB/s    in 0s      

2021-12-06 23:22:49 (38.0 MB/s) - written to stdout [1175/1175]

bash: line 25: [: -eq: unary operator expected

testing was performed by manually executing the script from the main repository directory and verifying that gef installed at the latest revision (a .gef-*.py was present), and that gef functioned within gdb.

Checklist

• My PR was done against the dev branch, not master.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required. -- N/A
• My change adds tests as appropriate. -- N/A
• I have read and agree to the CONTRIBUTING document.

[adi-g15]

It fixes this error (maybe warning :' ) both on an old ubuntu vm, and an updated arch 👍