Make alias handler and sq_quote_argv to check the return value of strbuf
APIs.

In sq_quote_argv() calls die(), but this fix handles strbuf failure as a
special case and returns to caller, since the caller - handle_alias()
also has to check the return value of other strbuf APIs and those checks
can be merged to one if() statement.
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20160510054725.6158.84597.stgit@devboxSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>