lib: fix warnings when building with clang

lib: fix warnings when building with clang

Building notmuch with CC=clang and CXX=clang++ produces the warnings:

CC -O2 lib/tags.o
lib/tags.c:43:5: warning: expression result unused [-Wunused-value]
    talloc_steal (tags, list);
    ^~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/talloc.h:345:143: note: expanded from:
  ...__location__); __talloc_steal_ret; })
                    ^~~~~~~~~~~~~~~~~~
1 warning generated.

CXX -O2 lib/message.o
lib/message.cc:791:5: warning: expression result unused [-Wunused-value]
    talloc_reference (message, message->tag_list);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/talloc.h:932:36: note: expanded from:
  ...(_TALLOC_TYPEOF(ptr))_talloc_reference_loc((ctx),(ptr), __location__)
     ^                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

Check talloc_reference() return value, and explicitly ignore
talloc_steal() return value as it has no failure modes, to silence the
warnings.