render: Fix -Wmaybe-uninitialized warning in RenderDrawLinesWithRects{,F}

The RenderDrawLinesWithRects and RenderDrawLinesWithRectsF functions can
sometimes call QueueCmdFillRects() with the data pointed to by frects
uninitialised. This can occur if none of the lines can be replaced with
rects, in which case the frects array is empty, and nrects is 0.

gcc 10.3.0 will detect this possibility, and print a warning like:
/home/david/Development/SDL/src/render/SDL_render.c: In function 'RenderDrawLinesWithRectsF':
/home/david/Development/SDL/src/render/SDL_render.c:2725:15: warning: '<unknown>' may be used uninitialized [-Wmaybe-uninitialized]
 2725 |     retval += QueueCmdFillRects(renderer, frects, nrects);
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/david/Development/SDL/src/render/SDL_render.c:499:1: note: by argument 2 of type 'const SDL_FRect *' to 'QueueCmdFillRects' declared here
  499 | QueueCmdFillRects(SDL_Renderer *renderer, const SDL_FRect * rects, const int count)
      | ^~~~~~~~~~~~~~~~~

This is harmless, because when this is uninitialised, nrects is always
0, so QueueCmdFillRects() does nothing anyway. We therefore can work
around this by only calling QueueCmdFillRects() when nrects is nonzero.
Somewhat impressively, gcc recognises that this is now safe.

src/render/SDL_render.c

@@ -2666,7 +2666,9 @@ RenderDrawLinesWithRects(SDL_Renderer * renderer,
         }
     }
 
-    retval += QueueCmdFillRects(renderer, frects, nrects);
+    if (nrects) {
+        retval += QueueCmdFillRects(renderer, frects, nrects);
+    }
 
     SDL_small_free(frects, isstack);
 
@@ -2721,7 +2723,9 @@ RenderDrawLinesWithRectsF(SDL_Renderer * renderer,
         }
     }
 
-    retval += QueueCmdFillRects(renderer, frects, nrects);
+    if (nrects) {
+        retval += QueueCmdFillRects(renderer, frects, nrects);
+    }
 
     SDL_small_free(frects, isstack);