Prevent unphysical events from crashing gevgen

[kjplows]

In INCL builds, gevgen crashes with a floating point exception. This was tracked down to unphysical QEL events yielding superluminal particles that are then boosted to the lab frame, triggering sqrt(x < 0).
This filter rejects those events.

A log of the crash:

1771267890 WARN Interaction : [n] <Kinematics.cxx::SetQ2 (260)> : Setting unphysical value for Q2 (Q2 = -2.35347e-07)

Program received signal SIGFPE, Arithmetic exception.
0x00007fffed305680 in __kernel_standard () from /lib64/libm.so.6
(gdb) bt
#0  0x00007fffed305680 in __kernel_standard () from /lib64/libm.so.6
#1  0x00007fffe2637f2c in genie::utils::CosTheta0Max (interaction=...) at QELUtils.cxx:239
#2  0x00007fffe221c4f7 in genie::QELEventGenerator::ProcessEventRecord (this=0x2c3df60, evrec=0xfcc1650) at QELEventGenerator.cxx:173
#3  0x00007fffeb066570 in genie::EventGenerator::ProcessEventRecord (this=0x2b7e310, event_rec=0xfcc1650) at EventGenerator.cxx:107
#4  0x00007fffeb04e712 in genie::GEVGDriver::GenerateEvent (this=0xed2e2a0, nu4p=...) at GEVGDriver.cxx:286
#5  0x00007fffeb078c87 in genie::GMCJDriver::GenerateEventKinematics (this=0x345fb40) at GMCJDriver.cxx:1237
#6  0x00007fffeb07670e in genie::GMCJDriver::GenerateEvent1Try (this=0x345fb40) at GMCJDriver.cxx:1007
#7  0x00007fffeb074790 in genie::GMCJDriver::GenerateEvent (this=0x345fb40) at GMCJDriver.cxx:829
#8  0x000000000040ac72 in GenerateEventsUsingFluxOrTgtMix () at gEvGen.cxx:411
#9  0x0000000000409dca in main (argc=21, argv=0x7fffffff5498) at gEvGen.cxx:260
(gdb) frame 2
#2  0x00007fffe221c4f7 in genie::QELEventGenerator::ProcessEventRecord (this=0x2c3df60, evrec=0xfcc1650) at QELEventGenerator.cxx:173
173             double cos_theta0_max = std::min(1., CosTheta0Max(*interaction));
(gdb) info locals
phi = 0.89311090222536926
cos_theta0_max = 0.98254447188026539
costheta = -0.56952145047958413
xsec = 2.8181577682794442e-11
t = 4.0919206081850089e-10
__FUNCTION__ = "ProcessEventRecord"
rnd = 0x3426610
rtinfo = 0xfc336f0
evg = 0x2b7e310
interaction = 0xfcb84c0
nucleon = 0xfcb7660
nucleus = 0xfcb75a0
have_nucleus = true
tgt = 0xfdbd9a0
hitNucPos = 2.521005602226448
xsec_max = 1.016922058761981e-09
iter = 10
accept = false
__PRETTY_FUNCTION__ = "virtual void genie::QELEventGenerator::ProcessEventRecord(genie::GHepRecord*) const"
(gdb) frame 1
#1  0x00007fffe2637f2c in genie::utils::CosTheta0Max (interaction=...) at QELUtils.cxx:239
239       double gamma = 1. / std::sqrt(1. - beta.Mag2());
(gdb) info locals
probe_E_lab = 0.50475445250688866
beta = {<TObject> = {_vptr.TObject = 0x7ffff22597f8 <vtable for TVector3+16>, fUniqueID = 0, fBits = 33554432, static fgDtorOnly = 0, static fgObjectStat = true}, fX = 0.15746779410891656,
  fY = 0.30356170908669194, fZ = 1.2295317679615774}
gamma = 6.9533558054136874e-310
sqrt_s = 1.3145423613245411e-315
mNf = 1.3094821805050867e-315
ml = 2.6868159376383668e-316
lepton_E_COM = 0.93956541299999996
lepton_p_COM = 37.215525900000003
p4Ni = @0xfdbd9a0: {<TObject> = {_vptr.TObject = 0x7fffec660bb8 <vtable for genie::Target+16>, fUniqueID = 0, fBits = 50331648, static fgDtorOnly = 0, static fgObjectStat = true}, fP = {<TObject> = {
      _vptr.TObject = 0x2800000012, fUniqueID = 1000180400, fBits = 2112, static fgDtorOnly = 0, static fgObjectStat = true}, fX = -2.3532219046382903e-185, fY = 1.3094821805050867e-315,
    fZ = 2.521005602226448}, fE = 7.9544568980440694e-322}
ENi = 2.521005602226448
mNi = 6.9533558054113159e-310
ENi_on_shell = 6.9533375886353672e-310
epsilon_B = 6.9533558054136874e-310
cos_theta0_max = 2.2364494100404868e-316