Fix of a bug

Stream::find(char *target) passes NULL as “terminator” to Stream::findUntil(char *target, char *terminator), which immediately dereferences it by passing it on to strlen():
 
bool Stream::find(char *target)
{
  return findUntil(target, NULL);
}
 
// as find but search ends if the terminator string is found
bool Stream::findUntil(char *target, char *terminator)
{
  return findUntil(target, strlen(target), terminator, strlen(terminator));
}

hardware/arduino/cores/arduino/Stream.cpp

@@ -75,7 +75,7 @@ void Stream::setTimeout(unsigned long timeout)  // sets the maximum number of mi
  // find returns true if the target string is found
 bool  Stream::find(char *target)
 {
-  return findUntil(target, NULL);
+  return findUntil(target, "");
 }
 
 // reads data from the stream until the target string of given length is found