of/base: Take NULL string into account for property with multiple strings

The current implementation just ignore any NULL string inserted in a
multiple strings property.
In some cases we can have a property with a fix number of strings but
not necessarily used, like for example in a list of valid pinmux modes.

 prop = "uart_rx", "uart_tx", "", "", "safe_mode";

Do no skip NULL string and take them into account in
of_property_read_string_index and of_property_count_strings.

Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

drivers/of/base.c

@@ -752,7 +752,7 @@ int of_property_read_string_index(struct device_node *np, const char *propname,
 
 	for (i = 0; total < prop->length; total += l, p += l) {
 		l = strlen(p) + 1;
-		if ((*p != 0) && (i++ == index)) {
+		if (i++ == index) {
 			*output = p;
 			return 0;
 		}
@@ -790,11 +790,9 @@ int of_property_count_strings(struct device_node *np, const char *propname)
 
 	p = prop->value;
 
-	for (i = 0; total < prop->length; total += l, p += l) {
+	for (i = 0; total < prop->length; total += l, p += l, i++)
 		l = strlen(p) + 1;
-		if (*p != 0)
-			i++;
-	}
+
 	return i;
 }
 EXPORT_SYMBOL_GPL(of_property_count_strings);