[bug] Nil pointer dereference in where clause

[edznux-dd]

Description of the bug

--- FAIL: TestRegressionNilDeref (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x61ff82]

Minimal reproduction payload

This payload has been minimified as much as possible in order to facilate debugging.
It is thus "abstract" (i.e: seems like an improbably SQL query). It can, however, as far as I understand, be injected in multiple place as part of a normal SQL query, and crash if the query isn't valid in the end.

func TestRegressionNilDeref(t *testing.T) {
	NewSelectBuilder().SQL("$0").Build()
}

I've openned a PR here: #204 for a proposed bug fix.