msm: Array bounds check for buffer index

Check the ASM buffer index boundary before using it
to access the buffer to avoid out of array bounds error.

CRs-Fixed: 2145996
Change-Id: I5492941dfd9e7c3fa90af6caec14f9ff7e75d248
Signed-off-by: Vignesh Kulothungan <vigneshk@codeaurora.org>