Do not crash if secontext is nullptr

LOG(DEBUG) tries to print a string pointed by secontext.get() but
crashed if it was nullptr.

Bug: 111888637
Test: "make -j50" and ran emulator
Change-Id: Iac78f650e7f48781030dc610f7d35cd52c250802
Signed-off-by: Roman Kiryanov <rkir@google.com>

vold_prepare_subdirs.cpp

@@ -63,7 +63,8 @@ static bool prepare_dir(struct selabel_handle* sehandle, mode_t mode, uid_t uid,
         secontext.reset(tmp_secontext);
     }
     LOG(DEBUG) << "Setting up mode " << std::oct << mode << std::dec << " uid " << uid << " gid "
-               << gid << " context " << secontext.get() << " on path: " << path;
+               << gid << " context " << (secontext ? secontext.get() : "null")
+               << " on path: " << path;
     if (secontext) {
         if (setfscreatecon(secontext.get()) != 0) {
             PLOG(ERROR) << "Unable to read setfscreatecon for: " << path;