Don't crash with a NULL thread name.

src/thread/beos/SDL_systhread.c

@@ -66,8 +66,9 @@ int
 SDL_SYS_CreateThread(SDL_Thread * thread, void *args)
 {
     /* The docs say the thread name can't be longer than B_OS_NAME_LENGTH. */
+    const char *threadname = thread->name ? thread->name : "SDL Thread";
     char name[B_OS_NAME_LENGTH];
-    SDL_snprintf(name, sizeof (name), "%s", thread->name);
+    SDL_snprintf(name, sizeof (name), "%s", threadname);
     name[sizeof (name) - 1] = '\0';
 
     /* Create the thread and go! */

src/thread/pthread/SDL_systhread.c

@@ -82,6 +82,7 @@ SDL_SYS_SetupThread(const char *name)
     int i;
     sigset_t mask;
 
+    if (name != NULL) {
 #if ( (__MACOSX__ && (MAC_OS_X_VERSION_MAX_ALLOWED >= 1060)) || \
       (__IPHONEOS__ && (__IPHONE_OS_VERSION_MAX_ALLOWED >= 30200)) )
         if (pthread_setname_np != NULL) { pthread_setname_np(name); }
@@ -90,6 +91,7 @@ SDL_SYS_SetupThread(const char *name)
 #elif HAVE_PTHREAD_SET_NAME_NP
         pthread_set_name_np(pthread_self(), name);
 #endif
+    }
 
     /* Mask asynchronous signals for this thread */
     sigemptyset(&mask);

src/thread/windows/SDL_systhread.c

@@ -161,8 +161,9 @@ typedef struct tagTHREADNAME_INFO
 void
 SDL_SYS_SetupThread(const char *name)
 {
-#if 0  /* !!! FIXME: __except needs C runtime, which we don't link against. */
-#ifdef _MSC_VER  /* !!! FIXME: can we do SEH on other compilers yet? */
+    if (name != NULL) {
+        #if 0  /* !!! FIXME: __except needs C runtime, which we don't link against. */
+        #ifdef _MSC_VER  /* !!! FIXME: can we do SEH on other compilers yet? */
         /* This magic tells the debugger to name a thread if it's listening. */
         THREADNAME_INFO inf;
         inf.dwType = 0x1000;
@@ -178,8 +179,9 @@ SDL_SYS_SetupThread(const char *name)
         {
             /* The program itself should ignore this bogus exception. */
         }
-#endif
-#endif
+        #endif
+        #endif
+    }
 }
 
 SDL_threadID