fbcmap: prevent memory overflow

Add bounds check before copying data to prevent
buffer overflow.

Change-Id: I47b9685b1ab13c4863fb6db62bbb9497a00b36da
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>
Bug: 19091590

Original commit: https://github.com/faux123/Nexus_5/commit/b70aba01edb9f31abe02bc5197dfd41c6b37723e