ICU-22775 fix StringSearch pattern CE mem mgmt

[markusicu]

• the main bug in Java initializePatternPCETable() was that the pcetablesize never got updated
• in C++, if the pattern CE loops encountered a failure, they didn't free the array (unlikely but possible near OOM)
• I think I made the memory management less convoluted
• for good measure, I used the "append" functions to add the null terminator, rather than rely on these functions having over-allocated by one for the real collation elements

Checklist

• Required: Issue filed: ICU-22775
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable