SDL_image: lbm: Fix Extra-Half-Brite usage.

SDL Commits
1 
From ce8ba6ecac4df4cd64a1c78775de47b7ba0e903b Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Sat, 15 Aug 2026 16:26:17 -0400
Subject: [PATCH] lbm: Fix Extra-Half-Brite usage.

If the caller loads an ILBM file with 64 colors, and either provided a
colormap with <= 32 colors or set the Extra_Half-Brite flag, then duplicate
the lower 32 entries to half-intensity versions in the upper half.

This also initializes colormap to zero unconditionally, in case this logic
would otherwise touch uninitialized stack memory. Now if there's a weird file
that didn't provide all 32 colors, the original and half-brite ones will just
be black instead of random data.

https://en.wikipedia.org/wiki/ILBM#Extra_Half-Brite

(Also an SDL_memset for an unrelated thing was changed to SDL_zero, since I
was in here anyhow.)
---
 src/IMG_lbm.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/IMG_lbm.c b/src/IMG_lbm.c
index 5da775a11..8c8b73456 100644
--- a/src/IMG_lbm.c
+++ b/src/IMG_lbm.c
@@ -142,7 +142,8 @@ SDL_Surface *IMG_LoadLBM_IO(SDL_IOStream *src )
 
     nbcolors = 0;
 
-    SDL_memset( &bmhd, 0, sizeof( BMHD ) );
+    SDL_zeroa(colormap);
+    SDL_zero(bmhd);
     flagHAM = 0;
     flagEHB = 0;
 
@@ -303,7 +304,7 @@ SDL_Surface *IMG_LoadLBM_IO(SDL_IOStream *src )
         /* The 32 last colors are the same but divided by 2 */
         /* Some Amiga pictures save 64 colors with 32 last wrong colors, */
         /* they shouldn't !, and here we overwrite these 32 bad colors. */
-        if ( (nbcolors==32 || flagEHB ) && (1<<nbplanes)==64 )
+        if ( (nbplanes == 6) && (flagEHB || nbcolors <= 32) )
         {
             nbcolors = 64;
             ptr = &colormap[0];