gb_proxy: Use TLVP_PRES_LEN instead of TLVP_PRESENT

With TLVP_PRESENT we only check if a tiven TLV/IE is present,
but don't verify that it's length matches our expectation.  This can
lead to out-of-bounds reads, so let's always use TLVP_PRES_LEN.

Change-Id: I1519cff0f6b2fe77f9a91eee17e0055d9df1bce6
2 files changed