fix(_ltrim_colon_completions): do compaction of sparse COMPREPLY

The existing implementation of "_ltrim_colon_completions()" assumes
that COMREPLY is a dense array, i.e., all the array subscripts are
within the range 0 .. ${#COMPREPLY[@]}-1.  However, the highest
assigned subcript i can be i >= ${#COMPREPLY[@]} when the array is
sparse.  In fact, COMPREPLY can be sparse in "_known_hosts_real()"
because it unsets an empty elements before calling
"_ltrim_colon_completions()".

Author: akinomyoga

bash_completion

@@ -515,10 +515,12 @@ _get_pword()
 #
 __ltrim_colon_completions()
 {
+    local i=${#COMPREPLY[*]}
+    ((i == 0)) && return 0
     if [[ $1 == *:* && $COMP_WORDBREAKS == *:* ]]; then
         # Remove colon-word prefix from COMPREPLY items
         local colon_word=${1%"${1##*:}"}
-        local i=${#COMPREPLY[*]}
+        COMPREPLY=("${COMPREPLY[@]}")
         while ((i-- > 0)); do
             COMPREPLY[i]=${COMPREPLY[i]#"$colon_word"}
         done