sys-file-reader: get_text_token did not set delimiter - bug #58943

parse_mrsets used loop local variables which were not initialized.
The regression worked with low compiler optimization levels because
the value for "delimiter" was kept over the loop iterations.
get_text_token did not set the delimiter value when only delimiters
are left in the string. I set the delimiter value to the last value
in the string in that case. I initialized the variables to see the
bug also in -O0.

Closes: https://savannah.gnu.org/bugs/?58943