Fix unwanted indentation in StdStream.print

When printing via `StdStream.print` strings containing the null
terminator, we were just printing the string until the null terminator
and replacing the unprinted characters by padding the printed string
with space characters.

This behavior made `String.size()` inconsistent with what `fprintf` was
really printing.

This behavior has been introduced in #1768, which ensure we respected
the buffer size.

Closes #4171

src/libponyrt/lang/stdfd.c

@@ -515,10 +515,8 @@ PONY_API size_t pony_os_stdin_read(char* buffer, size_t space)
 
 PONY_API void pony_os_std_print(FILE* fp, char* buffer, size_t len)
 {
-  if(len == 0)
-    fprintf(fp, "\n");
-  else
-    fprintf(fp, "%*.*s\n", (int)len, (int)len, buffer);
+  fwrite(buffer, len, 1, fp);
+  fprintf(fp, "\n");
 }
 
 PONY_API void pony_os_std_write(FILE* fp, char* buffer, size_t len)