Fix String.replace overlapping strcpy

Fixes #5949

Adds a test from the issue above and fixes the problem valgrind found.

Author: earlephilhower

cores/esp8266/WString.cpp

@@ -744,7 +744,7 @@ void String::replace(const String& find, const String& replace) {
             readFrom = foundAt + find.len();
             setLen(len() + diff);
         }
-        strcpy(writeTo, readFrom);
+        memmove(writeTo, readFrom, strlen(readFrom)+1);
     } else {
         unsigned int size = len(); // compute size needed for result
         while((foundAt = strstr(readFrom, find.buffer())) != NULL) {

tests/host/core/test_string.cpp

@@ -428,3 +428,15 @@ TEST_CASE("String SSO handles junk in memory", "[core][String]")
   REQUIRE(*s == "CO2_defect");
   s->~String();
 }
+
+
+TEST_CASE("Issue #5949 - Overlapping src/dest in replace", "[core][String]")
+{
+  String blah = "blah";
+  blah.replace("xx", "y");
+  REQUIRE(blah == "blah");
+  blah.replace("x", "yy");
+  REQUIRE(blah == "blah");
+  blah.replace(blah, blah);
+  REQUIRE(blah == "blah");
+}