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 3 minutes and 35 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #674179
build

00:03:35

passed #674184
build_android

00:01:01

passed #674180
build_clang_libcpp

00:03:21

passed #674183
build_clazy

00:02:59

passed #674182
build_mingw64_fedora

00:03:04

passed #674181
build_ubuntu_16_04

00:02:51