Backend fails when subslices are of different length

[Tabaie]

Code provided here.

Seems like somewhere in the backend it is assumed that all multi-dimensional slices have consistent lengths. i.e. If there is a witness variable Slice [][]frontend.Variable of length at least 2, it is assumed that len(Slice[0]) == len(Slice[1]).

One of the test routines provided to check this, TestInconsistentSliceLengthProving, fails the marshal/json subtest with panic: runtime error: index out of range [3] with length 3.

testing.tRunner.func1.2({0x102d1bc00, 0x1400002c7f8})
	go/1.19/libexec/src/testing/testing.go:1396 +0x1c8
testing.tRunner.func1()
	go/1.19/libexec/src/testing/testing.go:1399 +0x378
panic({0x102d1bc00, 0x1400002c7f8})
	go/1.19/libexec/src/runtime/panic.go:884 +0x204
github.com/consensys/gnark/internal/backend/bn254/witness.(*Witness).ToAssignment.func1.1(0x14000386940?, {0x102d5d4e0?, 0x14000396538?, 0x102d28b60?})
	project-path/gnark/internal/backend/bn254/witness/witness.go:147 +0x250
github.com/consensys/gnark/frontend/schema.parse({0x0?, 0x0, 0x0}, {0x14000386940?, 0x14000396538?}, {0x102d70ae8?, 0x102d5d4e0}, {0x1400039c030, 0x9}, {0x1400039c030, ...}, ...)
	project-path/gnark/frontend/schema/schema.go:223 +0x71c
github.com/consensys/gnark/frontend/schema.parse({0x14000399440?, 0x1, 0x1}, {0x14000386900?, 0x14000396530?}, {0x102d70ae8?, 0x102d5d4e0}, {0x1400039c019, 0x7}, {0x1400039c019, ...}, ...)
	project-path/gnark/frontend/schema/schema.go:341 +0xcf4
github.com/consensys/gnark/frontend/schema.parse({0x0?, 0x0, 0x0}, {0x140003868c0?, 0x14000396520?}, {0x102d70ae8?, 0x102d5d4e0}, {0x1400032df53, 0x5}, {0x1400032df53, ...}, ...)
	project-path/gnark/frontend/schema/schema.go:363 +0xa00
github.com/consensys/gnark/frontend/schema.parse({0x0?, 0x0, 0x0}, {0x14000386880?, 0x14000396520?}, {0x102d70ae8?, 0x102d5d4e0}, {0x0, 0x0}, {0x0, ...}, ...)
	project-path/gnark/frontend/schema/schema.go:292 +0x12c8
github.com/consensys/gnark/frontend/schema.Parse({0x14000386880?, 0x14000396520?}, {0x102d70ae8?, 0x102d5d4e0?}, 0x14000398ea0?)
	project-path/gnark/frontend/schema/schema.go:52 +0x80
github.com/consensys/gnark/internal/backend/bn254/witness.(*Witness).ToAssignment(0x1400000e8b8, {0x14000386880, 0x14000396520}, {0x102d70ae8?, 0x102d5d4e0?}, 0x0)
	project-path/gnark/internal/backend/bn254/witness/witness.go:161 +0xf0
github.com/consensys/gnark/backend/witness.(*Witness).toAssignment(0x14000187fe0, {0x14000386880, 0x14000396520}, {0x102d70ae8, 0x102d5d4e0})
	project-path/gnark/backend/witness/witness.go:239 +0xf8
github.com/consensys/gnark/backend/witness.(*Witness).MarshalJSON(0x14000187fe0)
	project-path/gnark/backend/witness/witness.go:162 +0x15c
github.com/consensys/gnark/test.(*Assert).marshalWitness(0x1400000e990, 0x14000187fe0, 0x1, 0x6?, {0x102acc5ac?, 0x18?, 0x102d65080?})
	project-path/gnark/test/assert.go:540 +0x78
github.com/consensys/gnark/test.(*Assert).ProverSucceeded.func1(0x1400000e990)
	project-path/gnark/test/assert.go:107 +0x58
github.com/consensys/gnark/test.(*Assert).Run.func1(0x1400013f380)
	project-path/gnark/test/assert.go:72 +0xe8
testing.tRunner(0x1400013f380, 0x1400000e960)
	go/1.19/libexec/src/testing/testing.go:1446 +0x10c
created by testing.(*T).Run
	go/1.19/libexec/src/testing/testing.go:1493 +0x300