As part of the refactoring in #5119, some vim_strchr() were changed to
strchr(). However, vim_strchr() behaves differently than strchr() when
c is NUL, returning NULL instead of a pointer to the NUL.
Revert the strchr() calls where it isn't known whether c is NUL, since
this causes a semantic change the surrounding code doesn't expect. In
the case of #6650, this led to a heap overrun.
Closes#6650
Hope this will make people using feed_command less likely: this hides bugs.
Already found at least two:
1. msgpackparse() will show internal error: hash_add() in case of duplicate
keys, though it will still work correctly. Currently silenced.
2. ttimeoutlen was spelled incorrectly, resulting in option not being set when
expected. Test was still functioning somehow though. Currently fixed.
It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.
