Handle another edge case in $PROMPT_COMMAND.

In particular: bash-preexec installs itself by appending
__bp_install_string to $PROMPT_COMMAND. That means that there's a pretty
good change that __bp_install_string will be the last thing in
$PROMPT_COMMAND when __bp_install is invoked.

If that happens, the $existing_prompt_command logic in __bp_install will
result in the pre-existing prompt command followed by `:` and no
trailing newline. That situation is not cleaned up by the two existing
 special cases.

Fix #153

bash-preexec.sh

@@ -328,8 +328,10 @@ __bp_install() {
     existing_prompt_command="${PROMPT_COMMAND:-}"
     # Edge case of appending to PROMPT_COMMAND
     existing_prompt_command="${existing_prompt_command//$__bp_install_string/:}" # no-op
-    existing_prompt_command="${existing_prompt_command//$'\n':$'\n'/$'\n'}" # remove known-token only
-    existing_prompt_command="${existing_prompt_command//$'\n':;/$'\n'}" # remove known-token only
+    # Remove some known no-op patterns.
+    existing_prompt_command="${existing_prompt_command//$'\n':$'\n'/$'\n'}"
+    existing_prompt_command="${existing_prompt_command//$'\n':;/$'\n'}"
+    existing_prompt_command="${existing_prompt_command%$'\n':}"
     __bp_sanitize_string existing_prompt_command "$existing_prompt_command"
     if [[ "${existing_prompt_command:-:}" == ":" ]]; then
         existing_prompt_command=