Fix AES-OFB support for errors, tests and self-test

Adds error handling into mbedtls_aes_crypt_ofb for AES errors, a self-test
for the OFB mode using NIST SP 800-38A test vectors and adds a check to
potential return errors in setting the AES encryption key in the OFB test
suite.
diff --git a/tests/Makefile b/tests/Makefile
index e68c1f3..86442c3 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -46,7 +46,7 @@
 endif
 
 APPS =	test_suite_aes.ecb$(EXEXT)	test_suite_aes.cbc$(EXEXT)	\
-	test_suite_aes.cfb$(EXEXT)		test_suite_aes.ofb$(EXEXT)	\
+	test_suite_aes.cfb$(EXEXT)	test_suite_aes.ofb$(EXEXT)	\
 	test_suite_aes.rest$(EXEXT)	\
 	test_suite_arc4$(EXEXT)		test_suite_asn1write$(EXEXT)	\
 	test_suite_base64$(EXEXT)	test_suite_blowfish$(EXEXT)	\
diff --git a/tests/suites/test_suite_aes.function b/tests/suites/test_suite_aes.function
index e1792dd..fc2cbac 100644
--- a/tests/suites/test_suite_aes.function
+++ b/tests/suites/test_suite_aes.function
@@ -316,7 +316,7 @@
     iv_len = unhexify( iv_str, hex_iv_string );
     in_buffer_len = unhexify( src_str, hex_src_string );
 
-    mbedtls_aes_setkey_enc( &ctx, key_str, key_len * 8 );
+    TEST_ASSERT( mbedtls_aes_setkey_enc( &ctx, key_str, key_len * 8 ) == 0 );
     src_str_next = src_str;
 
     while( in_buffer_len > 0 )