Fix heap overflow on computing parametrizations in non-shape-position case

[wegank]

In the following code taken from compute_parametrizations_non_shape_position_case:

msolve/src/fglm/fglm_core.c

Lines 1281 to 1288 in 44fbc68

	else{
	nmod_poly_fit_length(param->coords[nvars-2-nc],
	param->elim->length-1 );
	param->coords[nvars-2-nc]->length = data_bms->BMS->R1->length ;
	param->coords[nvars-2-nc]->coeffs[0] = 0;
	param->coords[nvars-2-nc]->coeffs[1] = 0;
	}

Line 1286 causes undefined behavior when param->elim->length is less than or equal to 2. This is in particular the case for cyclic5-16.ms when run with msolve -f cyclic5-16.ms -d 4 -P 2 -l 2 -t 1 -v 2: on one of my machines, this causes a segmentation fault.

This PR, as part of #214, fixes the issue.