Work around PETSc 1183 (RobotLocomotion#17136)

Work around some undefined-behavior sanitizer runtime failure in PETSc,
"applying zero offset to null pointer", in the BAIJ code, by checking
for some cases where a pointer might be NULL.

Reported as https://gitlab.com/petsc/petsc/-/issues/1183.

tools/workspace/petsc/patches/baij.patch

@@ -0,0 +1,38 @@
+diff --git a/src/mat/impls/baij/seq/baij.c b/src/mat/impls/baij/seq/baij.c
+index 7cf0f8e22a..c83e639fd7 100644
+--- src/mat/impls/baij/seq/baij.c
++++ src/mat/impls/baij/seq/baij.c
+@@ -1797,7 +1797,8 @@ PetscErrorCode MatGetValues_SeqBAIJ(Mat A,PetscInt m,const PetscInt im[],PetscIn
+     row = im[k]; brow = row/bs;
+     if (row < 0) {v += n; continue;} /* SETERRQ(PETSC_COMM_SELF,PETSC_ERR_ARG_OUTOFRANGE,"Negative row"); */
+     if (row >= A->rmap->N) SETERRQ1(PETSC_COMM_SELF,PETSC_ERR_ARG_OUTOFRANGE,"Row %D too large", row);
+-    rp   = aj + ai[brow]; ap = aa + bs2*ai[brow];
++    rp   = aj ? aj + ai[brow] : NULL;
++    ap   = aa ? aa + bs2*ai[brow] : NULL;
+     nrow = ailen[brow];
+     for (l=0; l<n; l++) { /* loop over columns */
+       if (in[l] < 0) {v++; continue;} /* SETERRQ(PETSC_COMM_SELF,PETSC_ERR_ARG_OUTOFRANGE,"Negative column"); */
+diff --git a/src/mat/impls/baij/seq/baij2.c b/src/mat/impls/baij/seq/baij2.c
+index e258605f8d..a15ab93b57 100644
+--- src/mat/impls/baij/seq/baij2.c
++++ src/mat/impls/baij/seq/baij2.c
+@@ -125,8 +125,8 @@ PetscErrorCode MatCreateSubMatrix_SeqBAIJ_Private(Mat A,IS isrow,IS iscol,MatReu
+     kstart   = ai[row];
+     kend     = kstart + a->ilen[row];
+     mat_i    = c->i[i];
+-    mat_j    = c->j + mat_i;
+-    mat_a    = c->a + mat_i*bs2;
++    mat_j    = c->j ? c->j + mat_i : NULL;
++    mat_a    = c->a ? c->a + mat_i*bs2 : NULL;
+     mat_ilen = c->ilen + i;
+     for (k=kstart; k<kend; k++) {
+       if ((tcol=ssmap[a->j[k]])) {
+@@ -138,7 +138,7 @@ PetscErrorCode MatCreateSubMatrix_SeqBAIJ_Private(Mat A,IS isrow,IS iscol,MatReu
+     }
+   }
+   /* sort */
+-  {
++  if (c->j && c->a) {
+     MatScalar *work;
+     ierr = PetscMalloc1(bs2,&work);CHKERRQ(ierr);
+     for (i=0; i<nrows; i++) {

tools/workspace/petsc/repository.bzl

@@ -13,6 +13,8 @@ def petsc_repository(
         build_file = "@drake//tools/workspace/petsc:package.BUILD.bazel",
         mirrors = mirrors,
         patches = [
+            # Cherry-picked from upstream (to be removed once we upgrade).
+            "@drake//tools/workspace/petsc:patches/baij.patch",
             # Cherry-picked from upstream (to be removed once we upgrade).
             "@drake//tools/workspace/petsc:patches/mal.patch",
             # Patch to fix dangerous global state in PETSc.