Fix double rounded by negative index
Before patched:
```
$ mruby -e 'p (-12..-1).map { |i| "Hello"[i] }.join'
"HelloHello"
```
After patched:
```
$ mruby -e 'p (-12..-1).map { |i| "Hello"[i] }.join'
"Hello"
```
MODIFY
src/string.c
src/string.c
@@ -449,9 +449,6 @@ str_substr(mrb_state *mrb, mrb_value str, mrb_int beg, mrb_int len)
if (clen == 0) {
len = 0;
}
- else if (beg < 0) {
- beg = clen + beg;
- }
if (beg > clen) return mrb_nil_value();
if (beg < 0) {
beg += clen;

Fix double rounded by negative index

- Before patched:
  ```
  $ mruby -e 'p (-12..-1).map { |i| "Hello"[i] }.join'
  "HelloHello"
  ```
- After patched:
  ```
  $ mruby -e 'p (-12..-1).map { |i| "Hello"[i] }.join'
  "Hello"
  ```