Fix get_ports.sh script

There were two errors:

1. get_random() function was returning random number with leading zeros
   that could lead the shell to interpret the number as octal value
   instead of decimal.  The surrounding whitespace was also causing
   problems.

2. The calculation of the port was off, it was adding the whole range
   and not just the min port to the base.

bin/tests/system/get_ports.sh

@@ -18,9 +18,10 @@ statefile=get_ports.state
 port_min=5001
 port_max=32767
 
-get_random() {
-    dd if=/dev/urandom bs=1 count=2 2>/dev/null | od -tu2 -An
-}
+get_random() (
+    # shellcheck disable=SC2005,SC2046
+    echo $(dd if=/dev/urandom bs=1 count=2 2>/dev/null | od -tu2 -An) | sed -e 's/^0*//'
+)
 
 get_port() {
     tries=10
@@ -36,7 +37,8 @@ get_port() {
 		    port="$1"
 		else
 		    port_range=$((port_max-port_min))
-		    port=$(($(get_random)%port_range+port_range))
+		    port_random=$(get_random)
+		    port=$((port_random%port_range+port_min))
 		fi
 	    fi