Fix regression in split_string_tok()

If there's no more separator we break early but dont update pos, so we go
into the code path that asserts we have reached the limit.

tests/checks/read.fish

@@ -389,3 +389,7 @@ echo foo | read status
 # CHECKERR: (Type 'help read' for related documentation)
 echo read $status
 # CHECK: read 2
+
+echo ' foo' | read -n 1 -la var
+set -S var
+#CHECK: $var: set in local scope, unexported, with 0 elements