Fix the mandrill.tiff regression (#145)

* fix mandrill.tiff regression from upstream

* use contains_key(), added StripTileTagConflict error

* updated test_div_zero, added test_no_rows_per_strip

src/error.rs

@@ -63,6 +63,7 @@ pub enum TiffFormatError {
     SignedIntegerExpected(Value),
     Format(String),
     RequiredTagEmpty(Tag),
+    StripTileTagConflict,
     #[doc(hidden)]
     /// Do not match against this variant. It may get removed.
     __NonExhaustive,
@@ -113,6 +114,7 @@ impl fmt::Display for TiffFormatError {
             }
             Format(ref val) => write!(fmt, "Invalid format: {:?}.", val),
             RequiredTagEmpty(ref val) => write!(fmt, "Required tag {:?} was empty.", val),
+            StripTileTagConflict => write!(fmt, "File should contain either (StripByteCounts and StripOffsets) or (TileByteCounts and TileOffsets), other combination was found."),
             __NonExhaustive => unreachable!(),
         }
     }