Fix warnings in gsm_rlcmac.cpp

1. ar.direction variable was not initialized
2. overrunning array "data->RLC_DATA" of 20 bytes at byte offset 22 using index "i" (which evaluates to 22)
diff --git a/src/csn1.cpp b/src/csn1.cpp
index 45cc848..7f64823 100644
--- a/src/csn1.cpp
+++ b/src/csn1.cpp
@@ -89,6 +89,7 @@
 {
   ar->remaining_bits_len  = remaining_bits_len;
   ar->bit_offset          = bit_offset;
+  ar->direction           = 0;
 }
 
 static const char* ErrCodes[] =