Fixes to buffer overlows from long command line args, and unchecked dyn_str return codes. Also light refactoring.


client/mysqldump.c:
  Bug#26346 stack + buffer overrun in mysqldump
mysql-test/r/mysqldump.result:
  Bug#26346 stack + buffer overrun in mysqldump
mysql-test/t/mysqldump.test:
  Bug#26346 stack + buffer overrun in mysqldump