completions/sqlmap: use `_command_exists`

Addresses bash-it/bash-it#1632

completion/available/sqlmap.completion.bash

@@ -1,4 +1,4 @@
-#!/bin/bash
+# shellcheck shell=bash
 
 # ---------------------------------------------------------------------------+
 #                                                                            |
@@ -7,15 +7,16 @@
 #                                                                            |
 # ---------------------------------------------------------------------------+
 
-if command -v sqlmap > /dev/null; then
+if _command_exists sqlmap
+then
 
-    _sqlmap()
+    function _sqlmap()
     {
         local cur prev
 
         COMPREPLY=()
-        cur=$(_get_cword)
+        cur="$(_get_cword)"
-        prev=$(_get_pword)
+        prev="$(_get_pword)"
 
         case $prev in