tbf: Check for NULL in name() (Coverity)

The gprs_rlcmac_tbf::name() method is generally used to generate log
messages. To avoid the need for an explicit NULL check for the tbf
and to get a consistent text if it is NULL, this commit adds a
NULL check to the method itself so that it can be called with this ==
NULL.

Fixes: Coverity CID 1304680, 1304681
Sponsored-by: On-Waves ehf

src/tbf.cpp

@@ -914,6 +914,9 @@ const char *tbf_name(gprs_rlcmac_tbf *tbf)
 
 const char *gprs_rlcmac_tbf::name() const
 {
+	if (this == NULL)
+		return "(no TBF)";
+
 	snprintf(m_name_buf, sizeof(m_name_buf) - 1,
 		"TBF(TFI=%d TLLI=0x%08x DIR=%s STATE=%s)",
 		m_tfi, tlli(),