Eliminate undefined-shift

Credit OSS Fuzz

Author: Cristy

coders/dpx.c

@@ -1158,7 +1158,7 @@ static Image *ReadDPXImage(const ImageInfo *image_info,ExceptionInfo *exception)
     SetPrimaryChromaticity((DPXColorimetric)
       dpx.image.image_element[n].colorimetric,&image->chromaticity);
     image->depth=dpx.image.image_element[n].bit_size;
-    if (image->depth > 32)
+    if ((image->depth == 0) || (image->depth > 32))
       ThrowReaderException(CorruptImageError,"ImproperImageHeader");
     samples_per_pixel=1;
     quantum_type=GrayQuantum;