Tentative fix for fdo#83937

One clear bug in the code, in my opinion, was that
PDFSigningPKCS7PasswordCallback() returned its argument as such. However, a
PK11PasswordFunc should return "a pointer to the password. This memory must
have been allocated with PR_Malloc or PL_strdup", says
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/SSL_functions/pkfnc.html
.

I could not test this fix fully before my hardware token decided to block
itself, thanks to too many wrong PIN attempts. Possibly it would work to even
just pass NULL for the password callback function and its argument to
NSS_CMSEncoder_Start(). After all, at least with the hardware token and
associated software that I tested with, the software itself pops up a dialog
asking for the PIN (password).

Change-Id: I85a8b2833cfdd1a1d7b7779016fefb71dd53ab80