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 13 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #432789
build

00:03:48

passed #432794
build_android

00:01:19

passed #432790
build_clang_libcpp

00:13:38

passed #432793
build_clazy

00:08:13

passed #432792
build_mingw64_fedora

00:05:38

passed #432791
build_ubuntu_16_04

00:02:22