message: Allow low iterator to initially be at ::end() in ProgramViewer::blockForAddress
diff: @@ -210,11 +210,7 @@ QTextBlock ProgramViewer::blockForAddress(unsigned long addr) const {
  unsigned long lineNumber = adjustedLineNumber;
  auto low = m_labelAddrOffsetMap.lower_bound(lineNumber);
  - if (low == m_labelAddrOffsetMap.end()) {
  - return QTextBlock();
  - }
  -
  - if (lineNumber < low->first && (low == m_labelAddrOffsetMap.begin())) {
  + if (low == m_labelAddrOffsetMap.begin() && lineNumber < low->first) {
  // The line number is less that the position of the first offset block; block is directly inferred from
  // linenumber.
  return document()->findBlockByNumber(adjustedLineNumber);
  