bugfix: infinite loop with unusual but valid pattern

The bug appears when the find() return pos1,pos2 = 1,0
In the specific case the split() function returns the whole string, like when the separator is not found.

Sample:
local s = "x -a -b -c"
print("ok", s:split(" %-",false))
print("infinite loop?", s:split(" -",false)) 
print("infinite loop?", s:split("[ ]-",false))
print("end")