ie: add macro for calculating an IE's length

All over the place we do "ie[1] + 2" for getting the IE length. It
is much clearer to use a macro to do this. The macro also checks
for NULL, and returns zero in this case.

src/ie.h

@@ -258,6 +258,9 @@ enum ie_rsn_akm_suite {
 	((akm == IE_RSN_AKM_SUITE_SAE_SHA256) || \
 	(akm == IE_RSN_AKM_SUITE_FT_OVER_SAE_SHA256))
 
+#define IE_LEN(ie) \
+	(ie) ? (ie)[1] + 2 : 0
+
 struct ie_tlv_iter {
 	unsigned int max;
 	unsigned int pos;