BUG/MINOR: tools: only reset argument start upon new argument

BUG/MINOR: tools: only reset argument start upon new argument

In issue #2995, Thomas Kjaer reported that empty argument position
reporting had been broken yet again. This time it was broken by this
latest fix: 2b60e54fb1 ("BUG/MINOR: tools: improve parse_line()'s
robustness against empty args"). It turns out that this fix is not
the culprit and it's in fact correct. The culprit was the original
commit of this series, 7e4a2f39ef ("BUG/MINOR: tools: do not create
an empty arg from trailing spaces"), which used to reset arg_start
to outpos for every new char in addition to doing it for every arg.
This resulted in the end of the line to be seen as always being in
error, thus reporting an incorrect position that the caller would
correct in a generic way designating the beginning of the line. It
didn't reveal prior to the upper fix above because the misassigned
value was almost not used by then.

Assigning the value before entering the loop fixes this problem and
doens't break the series of previous oss-fuzz reproducers. Hopefully
it's the last one again.

This must be backported to 3.2. Thanks to @tkjaer for reporting the
issue along with a reproducer.