[Fix] Possible out-of-bounds read with malformed IT / MPTM / XM files…

… using the SymMOD Echo plugin when modern tempo mode with an invalid rows per beat setting of 0 was used. This fix is a backport of the following:

Merged revision(s) 16484 from trunk/OpenMPT:
[Fix] Reject a rows per beat value of 0 when loading a file with modern tempo mode, as this would result in infinite tick length and thus undefined behaviour when trying to convert the tick length to int (found with afl++ + ubsan)
........


git-svn-id: https://source.openmpt.org/svn/openmpt/branches/OpenMPT-1.30@21607 56274372-70c3-4bfc-bfc3-4c3a0b034d27

soundlib/Sndfile.cpp

@@ -561,6 +561,8 @@ bool CSoundFile::CreateInternal(FileReader file, ModLoadingFlags loadFlags)
 	if(!m_nDefaultSpeed)
 		m_nDefaultSpeed = 6;
 
+	if(!m_nDefaultRowsPerBeat && m_nTempoMode == TempoMode::Modern)
+		m_nDefaultRowsPerBeat = 1;
 	if(m_nDefaultRowsPerMeasure < m_nDefaultRowsPerBeat)
 		m_nDefaultRowsPerMeasure = m_nDefaultRowsPerBeat;
 	LimitMax(m_nDefaultRowsPerBeat, MAX_ROWS_PER_BEAT);