Replace array_search that only uses a boolean to check whether the value

Replace array_search that only uses a boolean to check whether the value
exists with in_array (which returns just that boolean), and fix the
left over call to array_search to also check for NULL return.
This should fix the problems that Fredrik, me and other PHP 4.1 users
had with Marc's recent changes.

From PHP docs:
"Note:  Prior to PHP 4.2.0, array_search() returns NULL on failure instead of FALSE."

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@7547 7612ce4b-ef26-0410-bec9-ea0150e637f0