fix the use of uninitialized value in regcomp

fix the use of uninitialized value in regcomp

the num_submatches field of some ast nodes was not initialized in
tre_add_tag_{left,right}, but was accessed later.

this was a benign bug since the uninitialized values were never used
(these values are created during tre_add_tags and copied around during
tre_expand_ast where they are also used in computations, but nothing
in the final tnfa depends on them).