Fix WString optimization

PR esp8266#6573 introduced a corner case where a blind String() without any
initialization was in an in invalid state because the buffer and len
would not be updated properly.  Concatenating to the empty string could
cause a failure.

Now, set the default state in ::init() to SSO (which is what happened
before when we were using String(char *s="")) and fix the crash.

Author: earlephilhower

cores/esp8266/WString.cpp

@@ -129,10 +129,9 @@ String::~String() {
 // /*********************************************/
 
 inline void String::init(void) {
-    setSSO(false);
-    setCapacity(0);
+    setSSO(true);
     setLen(0);
-    setBuffer(nullptr);
+    wbuffer()[0] = 0;
 }
 
 void String::invalidate(void) {