rational: validate values when constructed with AVRational

`rational` is supposed to "fix signs" and "reduce" whenever its values are set,
but I neglected to do this when its values were set by an AVRational. Now it
will do so on both.

app/common/rational.cpp

@@ -7,6 +7,7 @@ rational::rational(const AVRational &r) :
   numer(r.num),
   denom(r.den)
 {
+  validateConstructor();
 }
 
 rational rational::fromDouble(const double &flt)
@@ -136,6 +137,22 @@ QString rational::toString() const
   return QStringLiteral("%1/%2").arg(QString::number(numer), QString::number(denom));
 }
 
+void rational::validateConstructor()
+{
+  if(denom != intType(0))
+    {
+      if(numer != intType(0))
+        {
+          fixSigns();
+          reduce();
+        }
+      else
+        denom = intType(0);
+    }
+  else
+    numer = intType(0);
+}
+
 //Assignment Operators
 
 const rational& rational::operator=(const rational &rhs)