Fix regex for extracting SHM_* values for libsysdecode

There was an additional + after the {6} which is apparently ignored by the
FreeBSD regex implementation but was giving me an error when compiling on
MacOS.

While changing this also make sure that tables.h is not created if mktables
fails. The current rule would create a partial tables.h which causes following
incremental builds to use that broken file and fail with an unrelated
compilation error or even succeed even though they shouldn't.

Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D17069