ITS: fix zero field seeding

Signed-off-by: Felix Schlepper <felix.schlepper@cern.ch>

Author: f3sch

Detectors/ITSMFT/ITS/tracking/src/TrackerTraits.cxx

@@ -1278,8 +1278,9 @@ track::TrackParCov TrackerTraits<nLayers>::buildTrackSeed(const Cluster& cluster
 
   float snp, q2pt, q2pt2;
   if (mIsZeroField) {
-    const float tgp = o2::gpu::CAMath::ATan2(y3 - y1, x3 - x1);
-    snp = sign * tgp / o2::gpu::CAMath::Sqrt(1.f + tgp * tgp);
+    const float dx = x3 - x1;
+    const float dy = y3 - y1;
+    snp = sign * dy / o2::gpu::CAMath::Hypot(dx, dy);
     q2pt = sign / track::kMostProbablePt;
     q2pt2 = 1.f;
   } else {