Fix compiler thinking tmpdir may be unitialised in emacsclient

Fix compiler thinking tmpdir may be unitialised in emacsclient

This fixes the following warning:

	emacsclient.c: In function ‘set_local_socket’:
	/usr/include/x86_64-linux-gnu/bits/string3.h:111:3: error:
		‘tmpdir’ may be used uninitialized in this function
		[-Werror=maybe-uninitialized]
	   return __builtin___stpcpy_chk (__dest, __src, __bos (__dest));
	   ^
	emacsclient.c:1197:17: note: ‘tmpdir’ was declared here
	     const char *tmpdir;

* lib-src/emacsclient.c (set_local_socket): Get rid of use_tmpdir
variable and instead use tmpdir being non-NULL as sign that it should
be used.