[Toybox] [PATCH] sed: fix substitution of empty capturing groups.

[enh]

The test for \N where N was larger than the number of capturing groups
in the regular expression was incorrect, and firing for cases such as
matching __(ARM_)?NR_([a-z]*) against __NR_read, where the first group is
empty (because it failed to match) but the second group did match "read".

Use regex_t's re_nsub for the error check, and treat rm_so == -1 as a
signal to just copy nothing into the result.

(Found trying to build minijail in AOSP.)
---
 tests/sed.test   | 11 +++++++++++
 toys/posix/sed.c | 10 +++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)
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