In regular expressions, repetition operators`{0,0}` and `{,0}` are valid, but they always match the zero-length string. For instance, `a{0,0}` and `[a-z]{0,0}` both match the zero-length string.
When the whole regular expression consists in one of these repetitions it caused a segfault during the evaluation of the regular expression because the `forward_code_ref` field for the root atom in the atom's tree was null.
Closes#2084.
Victor M. Alvarez
8616165472