Fix uninitialized tmp_charset in Data_Stream constructors

Initialize tmp_charset=0 in MySQL_Data_Stream and PgSQL_Data_Stream
constructors. This field was used uninitialized in
process_pkt_handshake_response() when calling proxy_debug(), causing
Valgrind errors about uninitialized values in vsnprintf.

lib/PgSQL_Data_Stream.cpp

@@ -256,6 +256,7 @@ PgSQL_Data_Stream::PgSQL_Data_Stream() {
 	encrypted = false;
 	switching_auth_stage = 0;
 	switching_auth_type = 0;
+	tmp_charset = 0;
 	x509_subject_alt_name = NULL;
 	ssl = NULL;
 	rbio_ssl = NULL;

lib/mysql_data_stream.cpp

@@ -261,6 +261,7 @@ MySQL_Data_Stream::MySQL_Data_Stream() {
 	switching_auth_type = AUTH_UNKNOWN_PLUGIN;
 	switching_auth_sent = AUTH_UNKNOWN_PLUGIN;
 	auth_in_progress = 0;
+	tmp_charset = 0;
 	x509_subject_alt_name=NULL;
 	ssl=NULL;
 	rbio_ssl = NULL;