Copy non set positions as spaces

st selection don't insert in the selection position whose value is not
set. This is correct for the positions in the end of the line, but cause
some problems in the beginning. For example echo -e 'a\tb' will print in the
screen:

a	b

but after selecting and copying in some place you get:

ab

because positions from 1 to 7 don't have any value. This patch deals all
positions without value as blank (even at the end of the line).
---
 st.c |   17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)