Fix crash when overcommit_memory is inaccessible (#10848)

When `/proc/sys/vm/overcommit_memory` is inaccessible, fp is NULL.
`checkOvercommit` will return -1 without setting err_msg, and then
the err_msg is used to print the log, crash the server.
Set the err_msg variables to Null when declaring it, seems safer.

And the overcommit_memory error log will print two "WARNING",
like `WARNING WARNING overcommit_memory is set to 0!`, this PR
also removes the second WARNING in `checkOvercommit`.

Reported in #10846. Fixes #10846. Introduced in #10636 (7.0.1)