Fix assorted compiler warnings.

sql/mysqld.cc:
  Do not declare max_page_size twice. If large pages support
  is enabled, the code expects the size in max_desired_page_size.
storage/innobase/include/ibuf0ibuf.h:
  Remove trailing comma. Only present in C99.
  
  Approved by: Vasil (via IRC)
storage/innobase/include/row0row.h:
  Remove trailing comma. Only present in C99.
  
  Approved by: Vasil (via IRC)
strings/my_vsnprintf.c:
  No need to assert the obvious.