[2369] Fix vector::erase() call in InputSource

The single-argument variant of vector::erase() deletes just one element
of the vector, not an entire range.  Tests have been fixed to check
that the range from start to the current buffer position is dropped
correctly.

src/lib/dns/inputsource.cc

@@ -107,7 +107,7 @@ InputSource::compact() {
     if (buffer_pos_ == buffer_.size()) {
         buffer_.clear();
     } else {
-        buffer_.erase(buffer_.begin() + buffer_pos_);
+        buffer_.erase(buffer_.begin(), buffer_.begin() + buffer_pos_);
     }
 
     buffer_pos_ = 0;