Segmentation fault when a polynomial is zero

[remyoudompheng]

The following file crashes msolve:

x,y,z
257
0,
x*x+y*y+z*z,
x+y+z,
x+y*z

Zero polynomials are not forbidden according to the README.
The issue happened while using Sage after substituting numerical values into a polynomial system.

% ./msolve -f mini -P 1
AddressSanitizer:DEADLYSIGNAL
=================================================================
==1377530==ERROR: AddressSanitizer: SEGV on unknown address (pc 0x7f33f722bbd6 bp 0x7ffd08854ed0 sp 0x7ffd08854d80 T0)
==1377530==The signal is caused by a READ memory access.
==1377530==Hint: this fault was caused by a dereference of a high value address (see register values below).  Disassemble the provided pc to learn which register was used.
    #0 0x7f33f722bbd6 in import_input_data (/tmp/msolve/src/neogb/.libs/libneogb-0.5.0.so+0x82bd6) (BuildId: 2be063c67d42a6ff905764b3bf2a9f481a8b8bf2)
    #1 0x557bf1ad1ad8 in msolve_trace_qq (/tmp/msolve/.libs/msolve+0x4cad8) (BuildId: e7c6c61f288cced0a2dc2cd7692a6431914aaca4)
    #2 0x557bf1adbfe8 in real_msolve_qq (/tmp/msolve/.libs/msolve+0x56fe8) (BuildId: e7c6c61f288cced0a2dc2cd7692a6431914aaca4)
    #3 0x557bf1ae159e in core_msolve (/tmp/msolve/.libs/msolve+0x5c59e) (BuildId: e7c6c61f288cced0a2dc2cd7692a6431914aaca4)
    #4 0x557bf1aecf4c in main (/tmp/msolve/.libs/msolve+0x67f4c) (BuildId: e7c6c61f288cced0a2dc2cd7692a6431914aaca4)
    #5 0x7f33f5b4c84f  (/usr/lib/libc.so.6+0x2384f) (BuildId: 2f005a79cd1a8e385972f5a102f16adba414d75e)
    #6 0x7f33f5b4c909 in __libc_start_main (/usr/lib/libc.so.6+0x23909) (BuildId: 2f005a79cd1a8e385972f5a102f16adba414d75e)
    #7 0x557bf1a91d24 in _start (/tmp/msolve/.libs/msolve+0xcd24) (BuildId: e7c6c61f288cced0a2dc2cd7692a6431914aaca4)

[ederc]

Will be fixed via #69 .