dsp: q6voice: Reset dma_buf pointer to NULL in case of failure

In case of voice_alloc_rtac_mem_map_table() failure no memory get
allocated for rtac but dma_buf pointer might get corrupted and may
point to some unmapped memory region. Since we are only doing NULL
pointer check for dma_buf in method is_rtac_memory_allocated() we
need to reset such pointer to NULL in failure case.

Change-Id: I5e81adad4e302620757375f87fa15344e07dccf8
Signed-off-by: Ajit Pandey <ajitp@codeaurora.org>