fix(wtf): fix for missing optarg

In the original code, the existence of $1 was not checked after
performing `shift`.  This would cause an error when `set -o nounset`
is set.  This commit adds a check.

Author: akinomyoga

completions/wtf

@@ -20,7 +20,9 @@ _wtf()
     while (($# > 0)); do
         if [[ $1 == -f ]]; then
             shift
-            db=$1
+            if (($# > 0)); then
+                db=$1
+            fi
             break
         fi
         shift