fixed a bug in String::operator+ and String::operator+=

there was enough space allocated for both strings & the null-terminator

String.cpp

@@ -51,7 +51,7 @@ String::String(String&& other) noexcept : String()   // init via default ctor so
 
 String String::operator+(const String& rhs) const
 {
-    auto size = this->size() + rhs.size() - 1;      // minus one null-terminator, we need only one
+    auto size = this->size() + rhs.size() + 1;      // plus a null-terminator
     auto data = new char[size];                     // make some space
     std::strcpy(data, this->c_str());               // copy data of this
     String ret;
@@ -62,7 +62,7 @@ String String::operator+(const String& rhs) const
 
 String& String::operator+=(const String& other)
 {
-    auto size = this->size() + other.size() - 1;     // minus one null-terminator, we need only one
+    auto size = this->size() + other.size() + 1;     // plus a null-terminator
     auto data = new char[size];
     std::strcpy(data, this->c_str());                // copy our data
     std::strcat(data, other.c_str());                // append data of the other string