fix byte_buffer copy and assignment ctor

lib/include/srslte/common/common.h

@@ -134,6 +134,9 @@ public:
     byte_buffer_t(const byte_buffer_t& buf)
     {
       bzero(buffer, SRSLTE_MAX_BUFFER_SIZE_BYTES);
+      msg = &buffer[SRSLTE_BUFFER_HEADER_OFFSET];
+      next = NULL;
+      // copy actual contents
       N_bytes = buf.N_bytes;
       memcpy(msg, buf.msg, N_bytes);
     }
@@ -143,6 +146,8 @@ public:
       if (&buf == this)
         return *this;
       bzero(buffer, SRSLTE_MAX_BUFFER_SIZE_BYTES);
+      msg = &buffer[SRSLTE_BUFFER_HEADER_OFFSET];
+      next = NULL;
       N_bytes = buf.N_bytes;
       memcpy(msg, buf.msg, N_bytes);
       return *this;