fix(printf): fctprintf() must not append null terminator

Fixes #39, references #19

printf.c

@@ -134,9 +134,11 @@ static inline void _out_char(char character, void* buffer, size_t idx, size_t ma
 static inline void _out_fct(char character, void* buffer, size_t idx, size_t maxlen)
 {
   (void)idx; (void)maxlen;
+  if (character) {
     // buffer is the output fct pointer
     ((out_fct_wrap_type*)buffer)->fct(character, ((out_fct_wrap_type*)buffer)->arg);
   }
+}
 
 
 // internal secure strlen

test/test_suite.cpp

@@ -69,7 +69,8 @@ TEST_CASE("fctprintf", "[]" ) {
   printf_idx = 0U;
   memset(printf_buffer, 0xCC, 100U);
   test::fctprintf(&_out_fct, nullptr, "This is a test of %X", 0x12EFU);
-  REQUIRE(!strcmp(printf_buffer, "This is a test of 12EF"));
+  REQUIRE(!strncmp(printf_buffer, "This is a test of 12EF", 22U));
+  REQUIRE(printf_buffer[22] == (char)0xCC);
 }