Add accessor to get buf from mbedtls_pem_context

Co-authored-by: Gilles Peskine <gilles.peskine@arm.com>
Signed-off-by: Glenn Strauss <gstrauss@gluelogic.com>
diff --git a/tests/suites/test_suite_pem.function b/tests/suites/test_suite_pem.function
index 947f1fb..b3d4810 100644
--- a/tests/suites/test_suite_pem.function
+++ b/tests/suites/test_suite_pem.function
@@ -40,12 +40,21 @@
     int ret;
     size_t use_len = 0;
     size_t pwd_len = strlen( pwd );
+    const unsigned char *buf;
 
     mbedtls_pem_init( &ctx );
 
     ret = mbedtls_pem_read_buffer( &ctx, header, footer, (unsigned char *)data,
                 (unsigned char *)pwd, pwd_len, &use_len );
     TEST_ASSERT( ret == res );
+    if( ret != 0 )
+        goto exit;
+
+    TEST_EQUAL( use_len, ctx.buflen );
+    use_len = 0;
+    buf = mbedtls_pem_get_buffer( &ctx, &use_len );
+    TEST_ASSERT( buf == ctx.buf );
+    TEST_EQUAL( use_len, ctx.buflen );
 
 exit:
     mbedtls_pem_free( &ctx );