libmsc: gsm340_gen_oa_sub() may return negative value

gsm340_gen_oa() returns a negative value if the output buffer that the
caller passes is too small, so we have to check the return value of this
function.

Fixes: CID 174178
Fixes: CID 174179
Change-Id: I47215d7d89771730a7f84efa8aeeb187a0911fdb
1 file changed