OpenCL: Fix heap buffer overflow

Bug message from AddressSanitizer:

    ==6158==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7fffe774b7fc at pc 0x555557086b54 bp 0x7fffffffcee0 sp 0x7fffffffced8
    READ of size 1 at 0x7fffe774b7fc thread T0
        #0 0x555557086b53 in tesseract::HistogramRect(Pix*, int, int, int, int, int, int*) ../../../../../src/ccstruct/otsuthr.cpp:163

Signed-off-by: Stefan Weil <sw@weilnetz.de>

src/opencl/openclwrapper.cpp

@@ -1998,7 +1998,7 @@ static void populateTessScoreEvaluationInputData(
     }
   }
 
-  input->pix = pixCreate(input->width, input->height, 1);
+  input->pix = pixCreate(input->width, input->height, 8 * input->numChannels);
 }
 
 struct TessDeviceScore {