Handle newlines better in sslerrorprinter.cpp

According to 4cedf30e99/crypto/err/err_prn.c (L20)
the ERR_print_errors_cb function already adds newlines to the end
of each line. So we can just append right away. We do remove the
last newline as normally error messages don't end with a newline.
Also we may return 0 instead of 1.

src/linux_tcp/sslerrorprinter.cpp

@@ -26,17 +26,11 @@ namespace AMQP {
  */
 int sslerrorprintercallback(const char *str, size_t len, void *ctx)
 {
-    // cast to ourselves
-    auto *self = static_cast<SslErrorPrinter*>(ctx);
-
-    // if this is not the first line, add a newline character
-    if (!self->_message.empty()) self->_message.push_back('\n');
-
-    // store the message
-    self->_message.append(str, len);
+    // Cast to ourselves and store the error line. OpenSSL adds a newline to every error line.
+    static_cast<SslErrorPrinter*>(ctx)->_message.append(str, len);
 
     // continue with the next message
-    return 1;
+    return 0;
 }
 
 /**
@@ -73,6 +67,9 @@ SslErrorPrinter::SslErrorPrinter(int retval)
         // collect all error lines
         OpenSSL::ERR_print_errors_cb(&sslerrorprintercallback, this);
 
+        // remove the last newline
+        if (!_message.empty() && _message.back() == '\n') _message.pop_back();
+
         // done
         break;