pgsql: Suppress a couple of compiler warnings seen with older gcc versi

Suppress a couple of compiler warnings seen with older gcc versions.

To wit,
bgworker.c: In function `RegisterDynamicBackgroundWorker':
bgworker.c:761: warning: `generation' might be used uninitialized in this function
dsm_impl.c: In function `dsm_impl_op':
dsm_impl.c:197: warning: control reaches end of non-void function

Neither of these represent actual bugs, but we may as well tweak the code
so that more compilers can tell that.  This won't change the generated code
on compilers that do recognize that the cases are unreachable.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/5f1ab4610102a73c124000788585c1af2a36284b

Modified Files
--------------
src/backend/postmaster/bgworker.c  |    2 +-
src/backend/storage/ipc/dsm_impl.c |    6 ++++--
2 files changed, 5 insertions(+), 3 deletions(-)