Replace malloc with calloc

- platform layer currently broken (not adapted yet)
- memmory_buffer_alloc too
diff --git a/tests/suites/test_suite_pem.function b/tests/suites/test_suite_pem.function
index 303e4fc..6a62bfe 100644
--- a/tests/suites/test_suite_pem.function
+++ b/tests/suites/test_suite_pem.function
@@ -23,7 +23,7 @@
     ret = mbedtls_pem_write_buffer( start, end, buf, buf_len, NULL, 0, &olen );
     TEST_ASSERT( ret == MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL );
 
-    check_buf = (unsigned char *) mbedtls_malloc( olen );
+    check_buf = (unsigned char *) mbedtls_calloc( 1, olen );
     TEST_ASSERT( check_buf != NULL );
 
     memset( check_buf, 0, olen );