Null pointer dereference in mrb_str_prepend

# PoC ```ruby String.new.prepend("") ``` # Cause This crash is caused by null dereference in https://github.com/mruby/mruby/blob/master/mrbgems/mruby-string-ext/src/string.c#L474 # Test ``` $ gdb -q --args ./bin/mruby test4.rb Reading symbols from ./bin/mruby...done. (gdb) r Starting program: /home/tunz/working/mruby/mruby/bin/mruby test4.rb Program received signal SIGSEGV, Segmentation fault. 0x0000000000464553 in mrb_str_prepend (mrb=0x1538010, self=...) at /home/tunz/working/mruby/mruby/mrbgems/mruby-string-ext/src/string.c:474 474 RSTR_PTR(s1)[len] = '\0'; ```