FIX: GCD for rational polynomials (#3980)

Use of modular results by Chinese remainder. Code was included in wrong way
that led to syntax error. Rare case, as usually fixed by heuristics.

lib/polyrat.gi

@@ -641,7 +641,7 @@ local G, P, l, m, i;
        = DegreeOfLaurentPolynomial(G)
     then
       P := G;
-      G := RPGcdCRT(t.modulo,t.gcd,t.prime,t.brci);
+      G := RPGcdCRT(P,t.modulo,t.gcd,t.prime,t.brci);
       t.modulo := t.modulo * t.prime;
       Info(InfoPoly,3,"gcd mod ",t.modulo," = ",G);
       if G = P  then
@@ -764,10 +764,7 @@ local brci,gcd,fam,fc,gc;
   if gcd=fail then
     # fall back to the original version:
     gcd:=RPIGcd(f,g);
-    if Length(gcd)>0 and not IsOne(gcd[Length(gcd)]) then
-      gcd:=gcd/gcd[Length(gcd)];
-    fi;
-    return gcd;
+    return StandardAssociate(gcd);
 
   fi;
   fc:=Minimum(fc[2],gc[2]);

tst/testbugfix/2020-04-22-Polyratgcd.tst

@@ -0,0 +1,20 @@
+# Rational Gcd, Case that uses Chinese Remainder.
+# reported by Willed de Graaf
+gap> x:= X( Rationals,"x" );;
+gap> f:= x^133-96*x^127-3000*x^121+347008*x^115+6954540*x^109-
+> 390498144*x^103-10087059752*x^97+9198644544*x^91+807579680022*x^85-
+> 6238300160224*x^79+23897742669240*x^73-58054582641408*x^67+
+> 97528246656332*x^61-117693609901728*x^55+103415475294888*x^49-
+> 65875222862272*x^43+29721301380081*x^37-9025704490176*x^31+
+> 1657660059648*x^25-139314069504*x^19;;
+gap> g:= 133*x^132-12192*x^126-363000*x^120+39905920*x^114+
+> 758044860*x^108-40221308832*x^102-978444795944*x^96+837076653504*x^90+
+> 68644272801870*x^84-492825712657696*x^78+1744535214854520*x^72-
+> 3889657036974336*x^66+5949223046036252*x^60-6473148544595040*x^54+
+> 5067358289449512*x^48-2832634583077696*x^42+1099688151062997*x^36-
+> 279796839195456*x^30+41441501491200*x^24-2646967320576*x^18;;
+gap> Gcd(f,g);
+x^114-58*x^108-3513*x^102+113708*x^96+5435185*x^90+14389578*x^84-545845377*x^7\
+8+3397296936*x^72-11212759701*x^66+23664901370*x^60-34296838499*x^54+351344756\
+28*x^48-25546738973*x^42+12949230518*x^36-4363433955*x^30+880398720*x^24-80621\
+568*x^18