Bend without DESIGNENERGY gives infinite loop

Summary

Steps to reproduce

Define a bend without design energy

REAL bend_radius = 1;
REAL bend_angle = 0.1;
TLA_B : SBEND, ELEMEDGE=0.0, ANGLE = 25.0 / 180 * PI, FMAPFN = "1DPROFILE1-DEFAULT", L = 2 * bend_radius * sin(bend_angle / 2.0),  HAPERT = 2.2;

What is the current bug behavior?

Infinite loop

What is the expected correct behavior?

Either:

Throw exception at parsing
Track through with design energy zero
Set design energy to initial energy of the beam

Relevant logs and/or screenshots

OPAL[2]> Phase space dump frequency 1000 and statistics dump frequency 1 w.r.t. the time step.
SBend [2]> ======================================================================
SBend [2]> ===== TLA_B ==========================================================
SBend [2]> ======================================================================
SBend [2]> TLA_B using file 1DPROFILE1-DEFAULT (1D Profile type 1)
Warning> Warning: bend design energy is zero. Treating as drift.
^C
Program received signal SIGINT, Interrupt.
0x00000000008f10ee in Bend2D::calculateRefTrajectory(double&, double&) () at /home/scratch/OPAL/OPAL-fork/OPAL-src/ippl/src/PETE/PETE.h:800
800	PETE_DefineBinary(operator*, (a * b), OpMultipply)
Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.212.el6.x86_64 infinipath-psm-3.0.1-115.1015_open.2.el6.x86_64 libnl-1.1.4-2.el6.x86_64 nss-pam-ldapd-0.7.5-20.el6_6.3.x86_64 zlib-1.2.3-29.el6.x86_64
(gdb) bt
#0  0x00000000008f10ee in Bend2D::calculateRefTrajectory(double&, double&) () at /home/scratch/OPAL/OPAL-fork/OPAL-src/ippl/src/PETE/PETE.h:800
#1  0x00000000008f2a4b in Bend2D::initialise(PartBunchBase<double, 3u>*, double&, double&) ()
    at /home/scratch/OPAL/OPAL-fork/OPAL-src/src/Classic/AbsBeamline/Bend2D.cpp:209
#2  0x000000000088ef8c in void OpalBeamline::visit<SBend>(SBend const&, BeamlineVisitor&, PartBunchBase<double, 3u>*) ()
    at /home/scratch/OPAL/OPAL-fork/OPAL-src/src/Elements/OpalBeamline.h:107
#3  0x000000000087a7ee in ParallelTTracker::visitBeamline(Beamline const&) ()

Cause

In Bend2D::calculateRefTrajectory and RBend3D::trackRefParticleThrough the stepSize is defined as follows:

    const double stepSize = betaGamma / gamma * Physics::c * dt;

without designenergy_m = 0, betaGamma = gamma = 0.

Here a minimal step size could be set.