README: prevent segmentation fault and mixture of printf/wprintf (signal11#470)

And a few other improvements.

Author: endolith

README.md

@@ -97,7 +97,7 @@ device spec. Writing data (`hid_write`) at random to your HID devices can break
 
 ```c
 #include <stdio.h> // printf
-#include <wchar.h> // wprintf
+#include <wchar.h> // wchar_t
 
 #include <hidapi.h>
 
@@ -117,22 +117,28 @@ int main(int argc, char* argv[])
 	// Open the device using the VID, PID,
 	// and optionally the Serial number.
 	handle = hid_open(0x4d8, 0x3f, NULL);
+	if (!handle) {
+		printf("Unable to open device\n");
+
+		hid_exit()
+ 		return 1;
+	}
 
 	// Read the Manufacturer String
 	res = hid_get_manufacturer_string(handle, wstr, MAX_STR);
-	wprintf(L"Manufacturer String: %s\n", wstr);
+	printf("Manufacturer String: %ls\n", wstr);
 
 	// Read the Product String
 	res = hid_get_product_string(handle, wstr, MAX_STR);
-	wprintf(L"Product String: %s\n", wstr);
+	printf("Product String: %ls\n", wstr);
 
 	// Read the Serial Number String
 	res = hid_get_serial_number_string(handle, wstr, MAX_STR);
-	wprintf(L"Serial Number String: (%d) %s\n", wstr[0], wstr);
+	printf("Serial Number String: (%d) %ls\n", wstr[0], wstr);
 
 	// Read Indexed String 1
 	res = hid_get_indexed_string(handle, 1, wstr, MAX_STR);
-	wprintf(L"Indexed String 1: %s\n", wstr);
+	printf("Indexed String 1: %ls\n", wstr);
 
 	// Toggle LED (cmd 0x80). The first byte is the report number (0x0).
 	buf[0] = 0x0;