Initialize socketName in a thread safe way

Use pthread_once() to initialize the variable only once.
It was possible that 2 threads were modifying the variable at the same
time.

The ThreadSanitizer trace was:
==================
WARNING: ThreadSanitizer: data race (pid=14864)
  Write of size 1 at 0x7f8803cccd2b by main thread:
    #0 getSocketName src/winscard_msg.c:101 (libpcsclite.so.1+0xa28c)
    #1 getSocketName src/winscard_msg.c:85 (libpcsclite.so.1+0xa28c)
    #2 SCardCheckDaemonAvailability src/winscard_clnt.c:3550 (libpcsclite.so.1+0x65cf)
    #3 SCardEstablishContext src/winscard_clnt.c:479 (libpcsclite.so.1+0x66a7)
    #4 main /tmp/pcsc/pcsc_demo.cpp:86 (pcsc_demo+0x1517)

  Previous write of size 1 at 0x7f8803cccd2b by thread T1:
    #0 getSocketName src/winscard_msg.c:101 (libpcsclite.so.1+0xa28c)
    #1 getSocketName src/winscard_msg.c:85 (libpcsclite.so.1+0xa28c)
    #2 SCardCheckDaemonAvailability src/winscard_clnt.c:3550 (libpcsclite.so.1+0x65cf)
    #3 SCardEstablishContext src/winscard_clnt.c:479 (libpcsclite.so.1+0x66a7)
    #4 do_statuschange /tmp/pcsc/pcsc_demo.cpp:12 (pcsc_demo+0x1264)
    #5 statuschange_thread /tmp/pcsc/pcsc_demo.cpp:53 (pcsc_demo+0x1264)

  Location is global '<null>' at 0x000000000000 (libpcsclite.so.1+0xfd2b)

  Thread T1 (tid=14867, running) created by main thread at:
    #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:1001 (libtsan.so.2+0x5e686)
    #1 start_thread(char const*) /tmp/pcsc/pcsc_demo.cpp:61 (pcsc_demo+0x1432)
    #2 main /tmp/pcsc/pcsc_demo.cpp:84 (pcsc_demo+0x14cc)

SUMMARY: ThreadSanitizer: data race src/winscard_msg.c:101 in getSocketName
==================
ThreadSanitizer: reported 1 warnings

Thanks to Stefan Ehmann for the bug report.

src/winscard_msg.c

@@ -82,26 +82,27 @@ THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #define member_size(type, member) sizeof(((type *)0)->member)
 
-char *getSocketName(void)
+static char SocketName[member_size(struct sockaddr_un, sun_path)];
+static pthread_once_t SocketName_init_control = PTHREAD_ONCE_INIT;
+static void SocketName_init(void)
 {
- static char socketName[member_size(struct sockaddr_un, sun_path)];
-
- if ('\0' == socketName[0])
- {
- /* socket name not yet initialized */
- char *socketNameEnv;
+ /* socket name not yet initialized */
+ char *socketNameEnv;
 
-  socketNameEnv = getenv("PCSCLITE_CSOCK_NAME");
-  if (socketNameEnv)
-  strncpy(socketName, socketNameEnv, sizeof(socketName));
-  else
-  strncpy(socketName, PCSCLITE_CSOCK_NAME, sizeof(socketName));
+ socketNameEnv = getenv("PCSCLITE_CSOCK_NAME");
+ if (socketNameEnv)
+ strncpy(SocketName, socketNameEnv, sizeof SocketName);
+ else
+ strncpy(SocketName, PCSCLITE_CSOCK_NAME, sizeof SocketName);
 
-  /* Ensure a NUL byte */
-  socketName[sizeof socketName -1] = '\0';
- }
+ /* Ensure a NUL byte */
+ SocketName[sizeof SocketName -1] = '\0';
+}
 
- return socketName;
+char *getSocketName(void)
+{
+ pthread_once(&SocketName_init_control, SocketName_init);
+ return SocketName;
 }
 
 /**