kwbimage: fixing the issue with proper return code checking

kwbimage: fixing the issue with proper return code checking

EVP_VerifyFinal would return one of three values:
1 if the data is verified to be correct;
0 if it is incorrect;
-1 if there is any failure in the verification process.

The varification in unpatched version is wrong, since it ignored
the return value of -1.

The bug allows a malformed signature to be treated as a good
signature rather than as an error. This issue affects the
signature checks on DSA ans ECDSA keys used with SSL/TLS.

This issue is similar to CVE-2008-5077, CVE-2009-0021,
CVE-2009-0025, CVE-2009-0046 ~ CVE-2009-0049.

Signed-off-by: Young Xiao <92siuyang@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>