fix(tiff): care with missing rowsperstrip (#5160)

According to the TIFF spec, TIFF files with a missing rowsperstrip
should assume the whole image is one strip.

Somewhat related (found in debugging): add a DASSERT to help debug if
fmath.h's round_to_multiple() is given an invalid denominator.

---------

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/include/OpenImageIO/fmath.h

@@ -200,6 +200,7 @@ floor2(int x) noexcept
 template <typename V, typename M>
 inline OIIO_HOSTDEVICE V round_to_multiple (V value, M multiple)
 {
+    OIIO_DASSERT(multiple > M(0));
     if (value >= 0)
         value += V(multiple) - 1;
     return value - (value % V(multiple));

src/tiff.imageio/tiffinput.cpp

@@ -1309,8 +1309,14 @@ TIFFInput::readspec(bool read_meta)
     m_rowsperstrip = -1;
     if (!m_spec.tile_width) {
         TIFFGetField(m_tif, TIFFTAG_ROWSPERSTRIP, &m_rowsperstrip);
-        if (m_rowsperstrip > 0)
+        if (m_rowsperstrip > 0) {
+            // Only set the attrib if a legit value was found in the file
             m_spec.attribute("tiff:RowsPerStrip", m_rowsperstrip);
+            m_rowsperstrip = std::min(m_rowsperstrip, m_spec.height);
+        } else {
+            // Default if not found is "one strip for the whole image"
+            m_rowsperstrip = m_spec.height;
+        }
     }
 
     // The libtiff docs say that only uncompressed images, or those with