lib: (strutils.c) fix unchecked lookahead in ul_parse_size()

lib: (strutils.c) fix unchecked lookahead in ul_parse_size()

If the numeric value provided to ul_parse_size() via @str
is a decimal with fractions only containing zeros, the logic
fails to identify the end of the string and goes to the label
'check_suffix' and will do an unchecked lookahead (*p + 1)
that will result in an out-of-bounds read.

This is because the logic only checks for null-termination
when a fraction has been parsed, i.e. a fraction not only
containing zeros.

To fix the issue, we implicitly check for null-termination
when we have finished parsing the fraction.

Reported-by: Yashashree Gund <yash_gund@live.com>
Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>