Verify success of BIO_new_file()

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any):

• When the file doesnt exists (--rsa-public-key-path), execution results in Segmentation fault #1471

• Brief description of code changes (suitable for use as a commit message):

Suggested fix crash when public/private key file does not exist - may not be th full fix as the issue didn't happen on my PC. However, the output of BIO_new_file() was not checked, which could lead to such crash in PEM_read_bio_xxxxKey() (depending on the library implementation).

In addition moved reading the private key file before prompting for the password, so the user will not have to enter password before the failure because file does not exist.

[bmah888]

Thanks for the PR! While I was setting up to test this, I realized that the original crash doesn't happen on my test setups either (FreeBSD 13 and Ubuntu 20). But I think having the added error handling is a good idea, as well as the workflow change for getting the passwd from the user.

[bmah888]

This looks great, let's get this merged.