dmatest: prevent memory leakage on error path in thread

When we fail to allocate memory for thread->srcs or thread->dsts and src_cnt or
dst_cnt great than 1 we leak memory on error path. This patch fixes the issue.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>