frequent crashes due to missing floating point exception handler

[Achim Schreiber]

Hello!

xfoil 6.99.dfsg-2 (xenial xerus standard installation) suffers from frequent crashes, always "SIGFPE: floating-point exception - erroneous arithmetic operation". This usually does happen during analysis (oper -> visc 100000 -> alfa), especially with "strange" airfoils.
But first of all it does seem to be a missing error handler, because a) Windows(tm) version from http://web.mit.edu/drela/Public/web/xfoil/XFOIL6.99.zip does not crash under same inputs and b) it also happens in a different simple setup (also only on xenial xerus, not Windows(tm)):

steps to reproduce:
1. start xfoil
2. enter "naca 0100" (this is an illegal NACA code)
result: xfoil crashes
expected: no crash, error message like "LEFIND: LE point not found. Continuing..." expected

n.b.: normal operation is possible with "normal" airfoils usually and results are reasonable.

I should be able to provide some example for a "strange" airfoil if needed.