iconv: Fix broken _vte_conv_utf8_strlen

(cherry picked from commit 3287a66f02edd3f26e5da708d1c3d36b3418a23c)
iconv: Fix a broken unittest

(cherry picked from commit 8a17c8ee4474b84414e5ed35a604ff3abfbe3fbe)
iconv: Fix the UTF-8 decoder when a NUL byte is encountered

https://bugzilla.gnome.org/show_bug.cgi?id=730220
(cherry picked from commit 95879ab9b4aa113d7877e4d080fbb7b403963f3b)