netrc: avoid NULL deref on weird input

[bagder]

A dynbuf that never gets populated might return a NULL, and Coverity could find a way through like that.

[testclutch]

Analysis of PR #17275 at 2fa8a076:

Test 1192 failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 2 different CI jobs (the link just goes to one of them).

Test 1199 failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test 1917 failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test 3017 failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test 564 failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test 700 failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 3 different CI jobs (the link just goes to one of them).

Test 701 failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 3 different CI jobs (the link just goes to one of them).

There are more failures, but that's enough from Gha.

Generated by Testclutch