seq: fix superfluous output line

seq: fix superfluous output line

Under certain circumstances seq prints an extra line when the output
format has custom format with characters following the printed numbers:

    $ seq -f "%g " 1000000 1000000
     1e+06
     1e+06

This is due to the "print_extra_number" logic using strings to determine
whether a 'extra number' is needed, but only one string was trimmed
when using a custom printf format.

Prompted by https://lists.gnu.org/r/coreutils/2019-08/msg00001.html

* NEWS: Mention fix.
* src/seq.c (print_numbers): Trim the 'x0_str' string before comparing
it to the previous 'x_str' string.
* tests/misc/seq-extra-number.sh: Add this scenario.
* tests/local.mk (all_tests): Add new test.