Pass a NULL instead of an empty string in the value_string/true_false_string

field of hf_rtp_payload_type.  This should fix the "values output" test failures
in the FreeBSD builder.

svn path=/trunk/; revision=13959

epan/dissectors/packet-rtp.c

@@ -887,7 +887,7 @@ proto_register_rtp(void)
 				"rtp.p_type",
 				FT_UINT8,
 				BASE_DEC,
-				"",
+				NULL,
 				0x7F,
 				"", HFILL
 			}