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

SDL Commits
1 
From 8f06a629aaf375df9001f8c06142e0002e8ba404 Mon Sep 17 00:00:00 2001
From: David Gow <[EMAIL REDACTED]>
Date: Fri, 30 Jul 2021 22:31:17 +0800
Subject: [PATCH] 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 | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/render/SDL_render.c b/src/render/SDL_render.c
index 305267da1e..d111e77a11 100644
--- a/src/render/SDL_render.c
+++ b/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);