[libc] Print "<NULL>" for wide-character NULL strings

The existing code intends to print NULL strings as "<NULL>" (for the
sake of debug messages), but the logic is incorrect when handling
wide-character strings.  Fix the logic and add applicable unit tests.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/vsprintf.c

 		} else if ( *fmt == 's' ) {
 			if ( length < &type_sizes[LONG_LEN] ) {
 				ptr = va_arg ( args, char * );
+				if ( ! ptr )
+					ptr = "<NULL>";
 			} else {
 				wptr = va_arg ( args, wchar_t * );
+				if ( ! wptr )
+					ptr = "<NULL>";
 			}
-			if ( ( ptr == NULL ) && ( wptr == NULL ) )
-				ptr = "<NULL>";
 		} else if ( *fmt == 'p' ) {
 			intptr_t ptrval;
 

src/tests/vsprintf_test.c

 	snprintf_ok ( 64, "PCI 00:1f.3", "PCI %02x:%02x.%x", 0x00, 0x1f, 0x03 );
 	snprintf_ok ( 64, "Region [1000000,3f000000)", "Region [%llx,%llx)",
 		      0x1000000ULL, 0x3f000000ULL );
+
+	/* Null string (used for debug messages) */
+	snprintf_ok ( 16, "<NULL>", "%s", ( ( char * ) NULL ) );
+	snprintf_ok ( 16, "<NULL>", "%ls", ( ( wchar_t * ) NULL ) );
 }
 
 /** vsprintf() self-test */