Possible to cause SEGFAULT in SEQUENCE

[kyrsjo]

A student ( @jonashei ) in our general accelerator physics course found a way to crash MAD-X with segfault when defining a SEQUENCE. This is a shortened version of the script:

test1 : SEQUENCE, L=1;
Q1 : MULTIPOLE, at=0;
ENDSEQUENCE;

test2comb : SEQUENCE, L=2;
test1, at=1, from=Q1;
ENDSEQUENCE;

BEAM, PARTICLE=proton, ENERGY=7000;
USE, SEQUENCE=test2comb;

The crash happened on version 5.06.01; the original version of the script crashed MAD-X on both MacOS and Linux, and the shortened version crashed it on Linux (MacOS was not tried).

The expected outcome would be to have an error message when encountering the bad input, and possibly a "clean" crash instead of a segfault.