Minor sed cleanups

[arichardson]

Summary

sed: Fix a UBSan warning and clean up a pedantic few compiler warnings

This avoids adding a zero offset to a NULL pointer, but written in such
a way that the compiler can generate the same code as before. While
touching this file also clean up a few signed/unsigned warnings.

usr.bin/sed: Speed up vector grow

Double the size when we run out of space instead of using 1K at a time.
I noticed this while debugging an infinite loop due to
https://github.com/google/sanitizers/issues/1371

usr.bin/sed: Add an assertion that REG_STARTEND

This was helpful while tracking down an ASan interceptor bug
(https://github.com/google/sanitizers/issues/1371).

[imp]

I think this is fine, though I'd be tempted to fix NEEDSP so that its invocations require a ';'.... though I was lazy and didn't look to see if that was possible...

usr.bin/sed/process.c

742

this looks odd w/o the ;