backport fixes for static analyzer warnings

Commits:
- 09837928d9
- 10fb88d027

Excluded changes with std::atomic (C++98 requirement)

modules/imgcodecs/src/grfmt_jpeg2000.cpp

@@ -179,7 +179,7 @@ bool  Jpeg2KDecoder::readData( Mat& img )
 {
     Ptr<Jpeg2KDecoder> close_this(this, Jpeg2KDecoder_close);
     bool result = false;
-    int color = img.channels() > 1;
+    bool color = img.channels() > 1;
     uchar* data = img.ptr();
     size_t step = img.step;
     jas_stream_t* stream = (jas_stream_t*)m_stream;

modules/imgcodecs/src/grfmt_png.cpp

@@ -226,7 +226,7 @@ bool  PngDecoder::readData( Mat& img )
     volatile bool result = false;
     AutoBuffer<uchar*> _buffer(m_height);
     uchar** buffer = _buffer.data();
-    int color = img.channels() > 1;
+    bool color = img.channels() > 1;
 
     png_structp png_ptr = (png_structp)m_png_ptr;
     png_infop info_ptr = (png_infop)m_info_ptr;

modules/imgcodecs/src/grfmt_pxm.cpp

@@ -208,7 +208,7 @@ bool PxMDecoder::readHeader()
 
 bool PxMDecoder::readData( Mat& img )
 {
-    int color = img.channels() > 1;
+    bool color = img.channels() > 1;
     uchar* data = img.ptr();
     PaletteEntry palette[256];
     bool   result = false;
@@ -225,7 +225,7 @@ bool PxMDecoder::readData( Mat& img )
     // create LUT for converting colors
     if( bit_depth == 8 )
     {
-        CV_Assert(m_maxval < 256);
+        CV_Assert(m_maxval < 256 && m_maxval > 0);
 
         for (int i = 0; i <= m_maxval; i++)
             gray_palette[i] = (uchar)((i*255/m_maxval)^(m_bpp == 1 ? 255 : 0));

modules/imgcodecs/src/grfmt_sunras.cpp

@@ -160,7 +160,7 @@ bool  SunRasterDecoder::readHeader()
 
 bool  SunRasterDecoder::readData( Mat& img )
 {
-    int color = img.channels() > 1;
+    bool color = img.channels() > 1;
     uchar* data = img.ptr();
     size_t step = img.step;
     uchar  gray_palette[256] = {0};