lib/file: Fix issue #71 - Electric Boogaloo (Out of bounds access)

Add size constraint to make sure string terminates at the correct
length. Cygwin appears to not put null bytes to terminate string when
reading from a file stream.

src/lib/file.cpp

@@ -225,7 +225,7 @@ ErrorOrVoid File::insertString(tiny_utf8::string string){
 
         this->fileStream.read(reinterpret_cast<char*>(buffer), this->size.value);
         
-        readData = tiny_utf8::string((char *) buffer);
+        readData = tiny_utf8::string((char *) buffer, 0, this->size.value);
         readData.insert(readData.begin()+this->cursorPosition, string);
 
         this->fileStream.seekg(0);