fg-util: fix a possible buffer overflow

If the string passed is of a huge size, then bytes_read can be
higher and can overflow "pos" to a small value. This can cause
a potential buffer overflow when "pos" is used again in sscanf.
Fix this by validating bytes_read before it is used.

CRs-Fixed: 1077693
Change-Id: I59d4472b49b67f481992867a34e6779a4589d035
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>