Fix copy-paste error in WebPImage::inject_VP8X height check

Line 773 checks width > 0 instead of height > 0. This was
introduced in commit bf151a1. When height is 0, the enforce
does not catch it and height - 1 wraps as an unsigned integer.

Change the check to validate height instead of width.

Author: MarkLee131

src/webpimage.cpp

@@ -770,7 +770,7 @@ void WebPImage::inject_VP8X(BasicIo& iIo, bool has_xmp, bool has_exif, bool has_
   data[6] = (w >> 16) & 0xFF;
 
   /* set height - stored in 24bits */
-  Internal::enforce(width > 0, Exiv2::ErrorCode::kerCorruptedMetadata);
+  Internal::enforce(height > 0, Exiv2::ErrorCode::kerCorruptedMetadata);
   uint32_t h = height - 1;
   data[7] = h & 0xFF;
   data[8] = (h >> 8) & 0xFF;