SW Renderer: fix invalid read if VSYNC hint is initially set with an empty value (eg "")

because:
- GeHint return a value pointer.
- SetHint free internally the pointer
- The -now invalid- pointer is re-read

==9363== Invalid read of size 1
==9363==    at 0x4946860: SW_CreateRenderer (SDL_render_sw.c:1044)
==9363==    by 0x48F0EC3: SDL_CreateRenderer_REAL (SDL_render.c:938)
==9363==    by 0x48C5921: SDL_CreateRenderer (SDL_dynapi_procs.h:332)
==9363==    by 0x401584: main (main.c:421)
==9363==  Address 0x9c24040 is 0 bytes inside a block of size 1 free'd
==9363==    at 0x484621F: free (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==9363==    by 0x494E403: SDL_free_REAL (SDL_malloc.c:5432)
==9363==    by 0x48A6153: SDL_SetHintWithPriority_REAL (SDL_hints.c:76)
==9363==    by 0x48A6254: SDL_SetHint_REAL (SDL_hints.c:101)

src/render/software/SDL_render_sw.c

@@ -1030,17 +1030,24 @@ SW_CreateRenderer(SDL_Window * window, Uint32 flags)
 {
     const char *hint;
     SDL_Surface *surface;
+    SDL_bool no_hint_set;
 
     /* Set the vsync hint based on our flags, if it's not already set */
     hint = SDL_GetHint(SDL_HINT_RENDER_VSYNC);
     if (!hint || !*hint) {
+        no_hint_set = SDL_TRUE;
+    } else {
+        no_hint_set = SDL_FALSE;
+    }
+
+    if (no_hint_set) {
         SDL_SetHint(SDL_HINT_RENDER_VSYNC, (flags & SDL_RENDERER_PRESENTVSYNC) ? "1" : "0");
     }
 
     surface = SDL_GetWindowSurface(window);
 
     /* Reset the vsync hint if we set it above */
-    if (!hint || !*hint) {
+    if (no_hint_set) {
         SDL_SetHint(SDL_HINT_RENDER_VSYNC, "");
     }