API: Bugfix: Use 0-terminated string in vim_strwidth

While the mb_string2cells function accepts a length parameter, it only seems to
work properly with 0-terminated strings, since valgrind reports a conditional
jump that depends on uninitialized values(means it reads after the string
boundaries which could result in overflows or wrong results)

src/nvim/api/vim.c

@@ -73,7 +73,10 @@ Integer vim_strwidth(String str, Error *err)
     return 0;
   }
 
-  return mb_string2cells((char_u *)str.data, (int)str.size);
+  char *buf = xstrndup(str.data, str.size);
+  Integer rv = mb_string2cells((char_u *)buf, -1);
+  free(buf);
+  return rv;
 }
 
 StringArray vim_list_runtime_paths(void)