Return -1 from our PEM password callback

Apparently, contrary to its documentation, this is how OpenSSL now
wants us to report an error.

Fixes bug 26116; bugfix on 0.2.5.16.

changes/bug26116

+  o Minor bugfixes (compatibility, openssl):
+    - Work around a change in OpenSSL 1.1.1 where
+      return values that would previously indicate "no password" now
+      indicate an empty password. Without this workaround, Tor instances
+      running with OpenSSL 1.1.1 would accept descriptors that other Tor
+      instances would reject. Fixes bug 26116; bugfix on 0.2.5.16.
+      

src/common/crypto.c

@@ -653,7 +653,12 @@ pem_no_password_cb(char *buf, int size, int rwflag, void *u)
   (void)size;
   (void)rwflag;
   (void)u;
-  return 0;
+  /* The openssl documentation says that a callback "must" return 0 if an
+   * error occurred.  But during the 1.1.1 series (commit c82c3462267afdbbaa5
+   * they changed the interpretation so that 0 indicates an empty password and
+   * -1 indicates an error. We want to reject any encrypted PEM buffers, so we
+   * return -1.  This will work on older OpenSSL versions and LibreSSL too. */
+  return -1;
 }
 
 /** Read a PEM-encoded private key from the <b>len</b>-byte string <b>s</b>