libiberty: Fix some demangler crashes caused by reading past end of input.

libiberty: Fix some demangler crashes caused by reading past end of input.

In various situations the cplus_demangle () function could read past the
end of input causing crashes. Add checks in various places to not advance
the demangle string location and fail early when end of string is reached.
Add various examples of input strings to the testsuite that would crash
test-demangle before the fixes.

Found by using the American Fuzzy Lop (afl) fuzzer.

libiberty/ChangeLog:

       * cplus-dem.c (demangle_signature): After 'H', template function,
       no success and don't advance position if end of string reached.
       (demangle_template): After 'z', template name, return zero on
       premature end of string.
       (gnu_special): Guard strchr against searching for zero characters.
       (do_type): If member, only advance mangled string when 'F' found.
       * testsuite/demangle-expected: Add examples of strings that could
       crash the demangler by reading past end of input.