UTF.cc: add 'max_ascii_chars' parameter to unicodeToAscii7()

which would allow to determine the smallest amount of
memory to safely allocate the indices for the converted
ascii string (returned in the out 6th parameter).

We set it with a default value of 2, as that precision is
enough for the use that poppler makes of unicodeToAscii7()
for the text search functionality.

See !327 for original bug report.
6 jobs for BUG_ascii7_underallocated in 6 minutes and 57 seconds (queued for 2 seconds)
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passed #443253
build

00:02:46

passed #443258
build_android

00:01:53

passed #443254
build_clang_libcpp

00:03:46

passed #443257
build_clazy

00:06:56

passed #443256
build_mingw64_fedora

00:02:41

passed #443255
build_ubuntu_16_04

00:02:52