fzf-tmux: validate attached layout size value

Attached option form (e.g. -l50) captured size via ${arg:2} with no
validation, unlike the separate-argument form. For -l/-u the value
reached $((...)) arithmetic, where a[$(cmd)] runs cmd during option
parsing, before any tmux check. Same value was also expanded unquoted
into `tmux popup ... $opt`, allowing tmux flag injection.

Apply the separate-form regex to the attached form too. Rejects the
array-subscript payload and space-bearing geometry values that enabled
flag injection.

Reported-by: sb24CK (https://github.com/sb24CK)

bin/fzf-tmux

@@ -78,6 +78,7 @@ while [[ $# -gt 0 ]]; do
       fi
       if [[ ${#arg} -gt 2 ]]; then
         size="${arg:2}"
+        [[ $size =~ ^[0-9%,]+$ || $size =~ ^[A-Z]$ ]] || continue
       else
         if [[ $1 =~ ^[0-9%,]+$ ]] || [[ $1 =~ ^[A-Z]$ ]]; then
           size="$1"