Check for NULL pointer in Print.write().

Otherwise, trying to print(NULL) or write(NULL) could print a random
character.

http://code.google.com/p/arduino/issues/detail?id=941

cores/arduino/Print.h

@@ -46,7 +46,10 @@ class Print
     void clearWriteError() { setWriteError(0); }
   
     virtual size_t write(uint8_t) = 0;
-    size_t write(const char *str) { return write((const uint8_t *)str, strlen(str)); }
+    size_t write(const char *str) {
+      if (str == NULL) return 0;
+      return write((const uint8_t *)str, strlen(str));
+    }
     virtual size_t write(const uint8_t *buffer, size_t size);
     
     size_t print(const __FlashStringHelper *);