Fix Uninitialized String offset error in validUtf8()

[pravussum]

No description provided.

[yunosh]

When does this happen? Do you have a test case?

[pravussum]

I made this change in reaction to a crash in Nextcloud Mail (using the Horde libraries under the hood) where the stack trace revealed this very line. I think its an edge case (It was a spam mail being processed).
After fixing it in-place, the crash didn't happen anymore, but I'm afraid I deleted the spam message.

[pravussum]

I have a backtrace now:

"file":"/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Util/Horde/String.php","line":873,"function":"onAll","class":"OC\\Log\\ErrorHandler","type":"::","args":[2,"Uninitialized string offset 14","/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Util/Horde/String.php",873]
"file":"/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Mime/Horde/Mime/Headers/Element.php","line":126,"function":"validUtf8","class":"Horde_String","type":"::","args":[null]
"file":"/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Mime/Horde/Mime/Headers/Addresses.php","line":86,"function":"_sanityCheck","class":"Horde_Mime_Headers_Element","type":"->","args":[null]
"file":"/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Mime/Horde/Mime/Headers/Element.php","line":79,"function":"_setValue","class":"Horde_Mime_Headers_Addresses","type":"->","args":[null]
"file":"/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Mime/Horde/Mime/Headers/Element.php","line":56,"function":"setValue","class":"Horde_Mime_Headers_Element","type":"->","args":[null]
"file":"/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Mime/Horde/Mime/Headers.php","line":201,"function":"__construct","class":"Horde_Mime_Headers_Element","type":"->","args":["Cc",null]
"file":"/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Mime/Horde/Mime/Headers.php","line":371,"function":"addHeader","class":"Horde_Mime_Headers","type":"->","args":["Cc",null]
"file":"/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Imap_Client/Horde/Imap/Client/Data/Fetch.php","line":628,"function":"parseHeaders","class":"Horde_Mime_Headers","type":"::","args":[{"__class__":"Horde_Stream_Temp","stream":null}]
"file":"/var/www/html/custom_apps/mail/vendor/pear-pear.horde.org/Horde_Imap_Client/Horde/Imap/Client/Data/Fetch.php","line":157,"function":"_getHeaders","class":"Horde_Imap_Client_Data_Fetch","type":"->","args":["0",1,3]