EAP-IKEv2: Try to make transform parser simpler to understand

EAP-IKEv2: Try to make transform parser simpler to understand

Use a local variable to try to make ikev2_parse_proposal() easier for
static analyzers to understand. Bounds checking in the loop is really
done by the ikev2_parse_transform() function, so the p->num_transforms
value itself is of no importance for that part and even that was already
implicitly limited in range.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>