Prompted by a gcc warning of

‘err’ may be used uninitialized in this function

I checked p12_parse and indeed, after the PKCS#12 rewrite of
101433dfb42b333e48427baf9dd58ac4787c9786

there are a number of error paths going to bailout (which prints gpg_strerror (err))
but not setting err.

Fix seems quite simple, albeit it turns out to be a bit noisy at the line level.