diag: Fix issue with accessing reallocated buffer

Diag driver reallocates its buffer when the data read from SMD is
greater than IN_BUF_SIZE. It uses the old pointer even after
reallocating the buffer resulting in dangling pointer access. This
patch fixes this issue.

Change-Id: I21322f161df51accd0464a47ab0cf469fbb2959f
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>