fix(printf): pass dummy buffer for printf to _vsnprintf instead of NULL

Fixes #15

printf.c

@@ -647,7 +647,8 @@ int printf(const char* format, ...)
 {
   va_list va;
   va_start(va, format);
-  const int ret = _vsnprintf(_out_char, NULL, (size_t)-1, format, va);
+  char buffer[1];
+  const int ret = _vsnprintf(_out_char, buffer, (size_t)-1, format, va);
   va_end(va);
   return ret;
 }

test/test_suite.cpp

@@ -40,18 +40,33 @@ namespace test {
 
 
 // dummy putchar
-void test::_putchar(char)
+static char   printf_buffer[100];
-{ }
+static size_t printf_idx = 0U;
+void test::_putchar(char character)
+{
+  printf_buffer[printf_idx++] = character;
+}
 
 
 
 TEST_CASE("printf", "[]" ) {
   char buffer[100];
 
+  printf_idx = 0U;
+  memset(printf_buffer, 0xCC, 100U);
   REQUIRE(test::printf("% d", 4232) == 5);
+  REQUIRE(!strcmp(printf_buffer, " 4232"));
+}
+
+
+TEST_CASE("snprintf", "[]" ) {
+  char buffer[100];
 
   test::snprintf(buffer, 100U, "%d", -1000);
   REQUIRE(!strcmp(buffer, "-1000"));
+
+  test::snprintf(buffer, 3U, "%d", -1000);
+  REQUIRE(!strcmp(buffer, "-1"));
 }