Description:
------------
"imap_open" function does not check arguments for non-empty values.
Empty arguments crash c-client (as c-client-2006k) with 
"php-cgi: IMAP toolkit crash: Unable to look up user name" followed by php-cgi crash as well.



Reproduce code:
---------------
<?php
echo 'start ';
if ( ($link = imap_open($dsn, $username, $password, $flags)) === false ) {
   echo 'cannt open';
}
echo ' finish';
?>

Expected result:
----------------
"imap_open" function should return FALSE in case of missing or empty any of REQUIRED arg.

Actual result:
--------------
#0  0xb23b5402 in __kernel_vsyscall ()
#1  0xb195c601 in raise () from /lib/libc.so.6
#2  0xb195de5c in abort () from /lib/libc.so.6
#3  0x08395881 in fatal ()
#4  0x0839ccce in myusername_full ()
#5  0x0839ce7c in myhomedir ()
#6  0x083fb667 in mh_path ()
#7  0x083fbf8a in mh_isvalid ()
#8  0x083fca02 in mh_valid ()
#9  0x083a2b98 in mail_valid ()
#10 0x083b0d1b in mail_open ()
#11 0x08157783 in php_imap_do_open (ht=4, return_value=0x8ba2a48, return_value_ptr=0x0, this_ptr=0x0, 
    return_value_used=1, persistent=0)
    at /var/tmp/portage/dev-lang/php-5.2.6-r2/work/php-5.2.6/ext/imap/php_imap.c:804
#12 0x08157923 in zif_imap_open (ht=4, return_value=0x8ba2a48, return_value_ptr=0x0, this_ptr=0x0, 
    return_value_used=1) at /var/tmp/portage/dev-lang/php-5.2.6-r2/work/php-5.2.6/ext/imap/php_imap.c:825
#13 0x0833b134 in zend_do_fcall_common_helper_SPEC (execute_data=0xbc55be40)
    at /var/tmp/portage/dev-lang/php-5.2.6-r2/work/php-5.2.6/Zend/zend_vm_execute.h:200
#14 0x08340d95 in ZEND_DO_FCALL_SPEC_CONST_HANDLER (execute_data=0xbc55be40)
    at /var/tmp/portage/dev-lang/php-5.2.6-r2/work/php-5.2.6/Zend/zend_vm_execute.h:1679
#15 0x0833ac79 in execute (op_array=0x8b9a568)
    at /var/tmp/portage/dev-lang/php-5.2.6-r2/work/php-5.2.6/Zend/zend_vm_execute.h:92
#16 0x08313b59 in zend_execute_scripts (type=8, retval=0x0, file_count=3)
    at /var/tmp/portage/dev-lang/php-5.2.6-r2/work/php-5.2.6/Zend/zend.c:1134
#17 0x082bbfe9 in php_execute_script (primary_file=0xbc560270)
    at /var/tmp/portage/dev-lang/php-5.2.6-r2/work/php-5.2.6/main/main.c:2007
#18 0x08394dec in main (argc=1, argv=0xbc560364)
    at /var/tmp/portage/dev-lang/php-5.2.6-r2/work/php-5.2.6/sapi/cgi/cgi_main.c:1919