fix BMP dimension check

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -580,13 +580,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 this.stream.Read(palette, 0, colorMapSize);
             }
 
-            // TODO: ReSharper tells this expression is always false, looks like he's pretty right about it:
-            if (this.infoHeader.Width > int.MaxValue || this.infoHeader.Height > int.MaxValue)
-            {
-                throw new ArgumentOutOfRangeException(
-                    $"The input bmp '{this.infoHeader.Width}x{this.infoHeader.Height}' is "
-                    + $"bigger then the max allowed size '{int.MaxValue}x{int.MaxValue}'");
-            }
+            this.infoHeader.VerifyDimensions();
         }
     }
 }

src/ImageSharp/Formats/Bmp/BmpInfoHeader.cs

@@ -157,5 +157,17 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
             dest = this;
         }
+
+        internal void VerifyDimensions()
+        {
+            const int MaximumBmpDimension = 65535;
+
+            if (this.Width > MaximumBmpDimension || this.Height > MaximumBmpDimension)
+            {
+                throw new InvalidOperationException(
+                    $"The input bmp '{this.Width}x{this.Height}' is "
+                    + $"bigger then the max allowed size '{MaximumBmpDimension}x{MaximumBmpDimension}'");
+            }
+        }
     }
 }