math.polynomials: update pdiff for no-integers-as-sequences (reported by rswarbrick)

basis/math/polynomials/polynomials-tests.factor

@@ -31,3 +31,5 @@ IN: math.polynomials.tests
 [ t ] [ { 1 1 1 } { 1 1 1 } p= ] unit-test
 [ { 0 0 } { 1 1 } ] [ { 1 1 1 1 } { 1 1 } pgcd ] unit-test
 
+[ { 10 200 3000 } ] [ { 1 10 100 1000 } pdiff ] unit-test
+

basis/math/polynomials/polynomials.factor

@@ -88,7 +88,7 @@ PRIVATE>
     [ V{ 0 } clone V{ 1 } clone ] 2dip swap (pgcd) [ >array ] bi@ ;
 
 : pdiff ( p -- p' )
-    dup length v* { 0 } ?head drop ;
+    dup length iota v* unclip drop ;
 
 : polyval ( x p -- p[x] )
     [ length swap powers ] [ nip ] 2bi v. ;