regex: fix OOB bug discovered through ./v -g -force-bounds-checking test vlib/regex/

[spytheman]

• regex: fix OOB bug discovered through ./v -g -force-bounds-checking vlib/regex/

[huly-for-github]

Connected to Huly®: V_0.6-23439

[penguindark]

OK, I had a look at this, your solution can work but to avoid further problems I suggest to replace re.group_stack.clear() at line 356 with:

// restore initial state of the stack
for mut x in re.group_stack {
    x = -1
}