tiny-printf: Avoid printing NULL strings

Add a check for NULL strings to avoid printing junk to the console.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>

lib/tiny-printf.c

@@ -108,10 +108,12 @@ int vprintf(const char *fmt, va_list va)
 				w--;
 			while (w-- > 0)
 				putc(lz ? '0' : ' ');
+			if (p) {
 				while ((ch = *p++))
 					putc(ch);
 			}
 		}
+	}
 
 abort:
 	return 0;