pd: Qualify modep pointer before use.
pd: Qualify modep pointer before use.

If UFP sends invalid VDM responses to the DFP its possible that modep
pointer may be NULL.  CL qualifies all uses of modep to guarantee that
these invalid responses don't cause samus_pd to crash.

BRANCH=samus
BUG=chromium:476773
TEST=manual,
1. Still successfully negotiate alternate mode (both DP & GFU) with
   hoho.
2. passes usbpd_DisplayPortSink autotest.

Change-Id: If4a611182b5e659c5534c2206132ef76d4e023db
Signed-off-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/265620
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>

akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,coreboot/chrome-ec,fourier49/BIZ_EC,md5555/ec,coreboot/chrome-ec,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,md5555/ec,coreboot/chrome-ec,md5555/ec,fourier49/BZ_DEV_EC,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,coreboot/chrome-ec,coreboot/chrome-ec