Fix memory leak on error in pkcs7_get_signers_info_set

mbedtls_x509_name allocates memory, which must be freed if there is a
subsequent error.

Credit to OSS-Fuzz (https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=53811).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/pkcs7.c b/library/pkcs7.c
index fc6dd33..e4238b6 100644
--- a/library/pkcs7.c
+++ b/library/pkcs7.c
@@ -387,7 +387,7 @@
 
     ret = pkcs7_get_signer_info( p, end_set, signers_set );
     if( ret != 0 )
-        return( ret );
+        goto cleanup;
     count++;
 
     mbedtls_pkcs7_signer_info *prev = signers_set;