Always reserve at least 1 element of storage for sparse matrices (bug #56232). * Sparse.h (SparseRep (void), SparseRep (octave_idx_type n), SparseRep (octave_idx_type nr, octave_idx_type nc, octave_idx_type nz = 1)): Rewrite constructors to always create valid d and r pointers with enough memory for 1 value. Always initialize nzmx to at least 1. * Sparse.cc (SparseRep::change_length): Check for nz == 0 and change value to 1 so that sparse array always maintains memory for at least one value. * amd.cc (Famd): Re-write failing BIST test that now works. * data.cc (Fnzmax): Document that nzmax will always return at least 1. * sparse.cc (Fspalloc): Document that 1 element will always be allocated.

--- a/libinterp/dldfcn/amd.cc	Mon May 06 20:25:15 2019 -0700
+++ b/libinterp/dldfcn/amd.cc	Wed May 08 15:18:32 2019 -0700
@@ -182,7 +182,7 @@
 %!shared A, A2, opts
 %! A = ones (20, 30);
 %! A2 = ones (30, 30);
-%!
+
 %!testif HAVE_AMD
 %! assert(amd (A2), [1:30]);
 %! opts.dense = 25;
@@ -190,7 +190,9 @@
 %! opts.aggressive = 1;
 %! assert(amd (A2, opts), [1:30]);
 
+%!testif HAVE_AMD
+%! assert (amd ([]), zeros (1,0))
+
 %!error <S must be a square matrix|was unavailable or disabled> amd (A)
 %!error amd (A2, 2)
-%!error <matrix S is corrupted|was unavailable or disabled> amd ([])
 */