Fix String::replace()

Fixes: https://github.com/espressif/arduino-esp32/issues/4920

cores/esp32/WString.cpp

@@ -744,6 +744,7 @@ void String::replace(const String& find, const String& replace) {
         }
     } else if(diff < 0) {
         char *writeTo = wbuffer();
+        unsigned int l = len();
         while((foundAt = strstr(readFrom, find.buffer())) != NULL) {
             unsigned int n = foundAt - readFrom;
             memmove(writeTo, readFrom, n);
@@ -751,9 +752,10 @@ void String::replace(const String& find, const String& replace) {
             memmove(writeTo, replace.buffer(), replace.len());
             writeTo += replace.len();
             readFrom = foundAt + find.len();
-            setLen(len() + diff);
+            l += diff;
         }
         memmove(writeTo, readFrom, strlen(readFrom)+1);
+        setLen(l);
     } else {
         unsigned int size = len(); // compute size needed for result
         while((foundAt = strstr(readFrom, find.buffer())) != NULL) {