Add some extra error output and code cleanup in an attempt to fix/debug
a rare random testsuite problem in check_warnings, where the exit code
from mysqltest is somehow corrupted inside mysql-test-run.pl.

include/my_global.h:
  Fix compiler warnings on some platforms.
mysql-test/lib/My/SafeProcess.pm:
  Move dereference of $? subprocess exit code closer to where it is generated,
  to make the code more robust and on the chance that this will fix the
  occasional problems in check_warnings we see in Buildbot.
mysql-test/mysql-test-run.pl:
  When check_warnings failed, also log the mysqld server for which it failed.
sql/sql_lex.cc:
  Fix compiler warning about possibly uninitialised value, by rewriting a for()
  loop that is always executed at least once into a do .. while() loop with an
  assert.
sql/table.cc:
  Fix compiler warning about uninitialised value.
storage/federatedx/ha_federatedx.cc:
  Fix uninitialised variable.
storage/maria/ma_delete.c:
  Fix compiler warning about uninitialised value.
storage/maria/ma_loghandler.c:
  Fix compiler warning about uninitialised value.
storage/myisam/ft_stopwords.c:
  Fix compiler warning.
storage/myisam/mi_write.c:
  Fix compiler warning about possibly uninitialised value, by rewriting a while()
  loop that is always executed at least once into a do .. while() loop with an
  assert.
storage/xtradb/btr/btr0cur.c:
  Fix compiler warning about possibly uninitialised value.
support-files/compiler_warnings.supp:
  Fix warning suppression to cover all cases in yassl.
vio/viossl.c:
  Fix compiler warning.