thread: Use a flexible array member for "array" in SDL_TLSData.

Avoids UBSan warning (among other similar ones in SDL_thread.c):
src/thread/SDL_thread.c:109:13: runtime error: index 1 out of bounds for type 'struct (unnamed struct at src/thread/SDL_thread_c.h:70:5)[1]'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/thread/SDL_thread.c:109:13

(cherry picked from commit f9395a766f8eb1b9428539b3c41ff2f86197b2f8)

src/thread/SDL_thread.c

@@ -99,7 +99,7 @@ bool SDL_SetTLS(SDL_TLSID *id, const void *value, SDL_TLSDestructorCallback dest
 
         oldlimit = storage ? storage->limit : 0;
         newlimit = (storage_index + TLS_ALLOC_CHUNKSIZE);
-        new_storage = (SDL_TLSData *)SDL_realloc(storage, sizeof(*storage) + (newlimit - 1) * sizeof(storage->array[0]));
+        new_storage = (SDL_TLSData *)SDL_realloc(storage, sizeof(*storage) + newlimit * sizeof(storage->array[0]));
         if (!new_storage) {
             return false;
         }

src/thread/SDL_thread_c.h

@@ -67,11 +67,18 @@ extern void SDL_RunThread(SDL_Thread *thread);
 typedef struct
 {
     int limit;
+#ifdef _MSC_VER
+#pragma warning(push)
+#pragma warning(disable : 4200) // Flexible array members (C99)
+#endif
     struct
     {
         void *data;
         void(SDLCALL *destructor)(void *);
-    } array[1];
+    } array[];
+#ifdef _MSC_VER
+#pragma warning(pop)
+#endif
 } SDL_TLSData;
 
 // This is how many TLS entries we allocate at once