thermal/drivers/tsens: Fix null pointer dereference

compute_intercept_slope() is called from calibrate_8960() (in tsens-8960.c)
as compute_intercept_slope(priv, p1, NULL, ONE_PT_CALIB) which lead to null
pointer dereference (if DEBUG or DYNAMIC_DEBUG set).
Fix this bug by adding null pointer check.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: dfc1193d ("thermal/drivers/tsens: Replace custom 8960 apis with generic apis")
Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240411114021.12203-1-amishin@t-argos.ru