Make many counts of tracks and channels unsigned...

... And in some places where a library uses signed types, assert that
the reported number is not negative.

What led me to this, is that there are many places where a size_t value for
an allocation is the product of a number of channels and some other number.

src/import/ImportPCM.cpp

@@ -109,7 +109,7 @@ public:
 
 private:
    SFFile                mFile;
-   SF_INFO               mInfo;
+   const SF_INFO         mInfo;
    sampleFormat          mFormat;
 };
 
@@ -194,6 +194,8 @@ PCMImportFileHandle::PCMImportFileHandle(wxString name,
    mFile(std::move(file)),
    mInfo(info)
 {
+   wxASSERT(info.channels >= 0);
+
    //
    // Figure out the format to use.
    //
@@ -432,6 +434,8 @@ int PCMImportFileHandle::Import(TrackFactory *trackFactory,
 
       // PRL:  guard against excessive memory buffer allocation in case of many channels
       using type = decltype(maxBlockSize);
+      if (mInfo.channels < 1)
+         return eProgressFailed;
       auto maxBlock = std::min(maxBlockSize,
          std::numeric_limits<type>::max() /
             (mInfo.channels * SAMPLE_SIZE(mFormat))
@@ -440,6 +444,7 @@ int PCMImportFileHandle::Import(TrackFactory *trackFactory,
          return eProgressFailed;
 
       SampleBuffer srcbuffer;
+      wxASSERT(mInfo.channels >= 0);
       while (NULL == srcbuffer.Allocate(maxBlock * mInfo.channels, mFormat).ptr())
       {
          maxBlock >>= 1;