Allocate big enough buffer for text conversion

Buffer for conversion from Unicode to ASCII7 can
be small in some cases leading to invalid writes
which can result in crash.
This commit increases size of the buffer.
6 jobs for invalid-write-in-unicodeToAscii7 in 6 minutes and 2 seconds (queued for 18 seconds)
Status Job ID Name Coverage
  Build
passed #1082857
build

00:03:20

passed #1082862
build_android

00:00:51

passed #1082858
build_clang_libcpp

00:02:48

passed #1082861
build_clazy

00:03:32

passed #1082860
build_mingw64_fedora

00:03:44

passed #1082859
build_ubuntu_16_04

00:01:40