stop polyroots segfaulting on degree-0 polynomial

Basic/Math/cpoly.c

@@ -210,6 +210,9 @@ char *cpoly(double opr[], double opi[], int degree,
      the zerofinder will work provided the overflowed quantity is
      replaced by a large number. */
 
+  if (degree < 1)
+    return "algorithm only works for degree >= 1.";
+
   if (opr[0] == 0.0 && opi[0] == 0.0)
     return "algorithm fails if the leading coefficient is zero.";
 

Basic/Math/math.pd

@@ -362,7 +362,8 @@ EOF
 =for ref
 
 Complex roots of a complex polynomial, given coefficients in order
-of decreasing powers. As of 2.086, works with native-complex data.
+of decreasing powers. Only works for degree >= 1.
+As of 2.086, works with native-complex data.
 
 =for usage
 

t/math.t

@@ -53,6 +53,8 @@ $pa->inplace->erfi;
 ok( all( approx( $pa, pdl(0.00886,0.0) )), "erfi inplace" );
 }
 
+eval {polyroots(1,0)};
+like $@, qr/only works/, 'polyroots(1,0) throws exception not segfault';
 my $coeffs = pdl(cdouble, 1,-55,1320,-18150,157773,-902055, 3416930,-8409500,12753576,-10628640,3628800);
 my $roots = 1+sequence(10);
 my $got;