diag: Check buffer flush status before processing

There is a possibility of race condition between
drain_apps_data and process_apps_data for flushing
the apps data buffer to mux, Which can cause flushing
buffer twice and can lead to use-after-free issue.
The patch adds checks for buffer flush status before
using the same buffer for processing apps data.

Change-Id: I2cfe67304e73bcb4004884b986ac2bec44b29ba1
Signed-off-by: Hardik Arya <harya@codeaurora.org>