Fix undefined behavior in LDLT

When m==n the failed_rect.data() is nullptr, but then we still
subtract some small integer from it. Doing arithmetic on a null
pointer is undefined behavior. Clang's undefined Behavior sanitizer
says ldlt_app.cxx:2420:38: runtime error: applying non-zero offset
18446744073709551536 to null pointer

The copy_failed_rect ends up being a no-op because m==rfrom, but it's
still UB to do arithmetic on nullptr, even if never de-referenced.

src/ssids/cpu/kernels/ldlt_app.cxx

@@ -2415,10 +2415,10 @@ class LDLT {
             // Rectangular part
             // (be careful with blocks that contain both diag and rect parts)
             copy_failed_rect(
-                  get_nrow(nblk-1, m, block_size), get_ncol(jblk, n, block_size),
-                  get_ncol(nblk-1, n, block_size), cdata[jblk],
-                  failed_rect.data() + (jfail*(m-n)+(nblk-1)*block_size-n), m-n,
-                  &a[jblk*block_size*lda+(nblk-1)*block_size], lda
+                  get_nrow(nblk-1, m, block_size) - get_nrow(nblk-1, n, block_size),
+                  get_ncol(jblk, n, block_size), 0, cdata[jblk],
+                  failed_rect.data() + jfail*(m-n), m-n,
+                  &a[jblk*block_size*lda+n], lda
                   );
             for(int iblk=nblk; iblk<mblk; ++iblk) {
                copy_failed_rect(