gas: terminate buffer SB in do_repeat()

PR gas/31903

While elsewhere having realized that "one" doesn't point to a nul-
terminated string, it somehow didn't occur to me that the pre-existing
strstr() could have been wrong, and hence I blindly added a new use of
the function. Add the (already prior to 1e3c8144 ["gas: extend \+
support to .rept"]) missing call to sb_terminate(), leveraging that to
simplify the other two places where the lack of nul termination was
previously worked around.