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Avoid adding offsets to NULL chroma pointers in 4:0:0

Doing so is technically undefined behavior even though the pointers
are never dereferenced.

--- a/src/decode.c

+++ b/src/decode.c

@@ -2944,14 +2944,19 @@

         }

     }

-    // init loopfilter pointers

+    /* Init loopfilter pointers. Increasing NULL pointers is technically UB,

+     * so just point the chroma pointers in 4:0:0 to the luma plane here to

+     * avoid having additional in-loop branches in various places. We never

+     * dereference those pointers so it doesn't really matter what they

+     * point at, as long as the pointers are valid. */

+    const int has_chroma = f->cur.p.layout != DAV1D_PIXEL_LAYOUT_I400;

     f->lf.mask_ptr = f->lf.mask;

     f->lf.p[0] = f->cur.data[0];

-    f->lf.p[1] = f->cur.data[1];

-    f->lf.p[2] = f->cur.data[2];

+    f->lf.p[1] = f->cur.data[has_chroma ? 1 : 0];

+    f->lf.p[2] = f->cur.data[has_chroma ? 2 : 0];

     f->lf.sr_p[0] = f->sr_cur.p.data[0];

-    f->lf.sr_p[1] = f->sr_cur.p.data[1];

-    f->lf.sr_p[2] = f->sr_cur.p.data[2];

+    f->lf.sr_p[1] = f->sr_cur.p.data[has_chroma ? 1 : 0];

+    f->lf.sr_p[2] = f->sr_cur.p.data[has_chroma ? 2 : 0];

     f->lf.tile_row = 1;

     dav1d_cdf_thread_wait(&f->in_cdf);