printf(): Print null pointers (arg for %s = 0) as (null), fixes #4

src/console.asm

@@ -359,6 +359,21 @@ printf:
 
 		;--- Insert string to buffer ---;
 		.insertString:
+			test	rsi, rsi
+			jnz	.doInsertString
+			;rsi = NULL:
+			mov	r9b, 3
+			cmp	r10, printfBuffLen-7
+			je	.flushBuffer
+			.flushReturn_3:
+			mov	byte [printfBuff + r10], '('
+			mov	qword [printfBuff + r10 + 1], 'null'
+			mov	byte [printfBuff + r10 + 5], ')'
+			add	r10, 6
+			add	r11, 6
+			jmp	.endInsertString
+
+			.doInsertString:
 			cmp	byte [rsi], EOS
 			je	.endInsertString
 			mov	r9b, 1
@@ -409,6 +424,8 @@ printf:
 	je	.flushReturn_1
 	cmp	r9b, 2
 	je	.flushReturn_2
+	cmp	r9b, 3
+	je	.flushReturn_3
 
 	.wrapup:
 	mov	rax, NR_write