fix(printf): use C conform `NULL` instead of C++ `nullptr`

Fixes #14

README.md

@@ -52,10 +52,6 @@ int snprintf(char* buffer, size_t count, const char* format, ...);
 int vsnprintf(char* buffer, size_t count, const char* format, va_list va);
 ```
 
-If `buffer` is set to `nullptr` nothing is written and just the formatted length is returned.
-```C
-int length = sprintf(nullptr, "Hello, world"); // length is set to 12
-```
 
 **Due to genaral security reasons it is highly recommended to prefer and use `snprintf` (with the max buffer size as `count` parameter) instead of `sprintf`.**  
 `sprintf` has no buffer limitation, so when needed - use it really with care!
@@ -136,6 +132,11 @@ Notice that a value equal or larger than `count` indicates a truncation. Only wh
 the string has been completely written.  
 If any error is encountered, `-1` is returned.
 
+If `buffer` is set to `NULL` (`nullptr`) nothing is written and just the formatted length is returned.
+```C
+int length = sprintf(NULL, "Hello, world"); // length is set to 12
+```
+
 
 ## Compiler switches/defines
 

printf.c

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