IntegerMatcher: Fix division by zero

Credit to OSS-Fuzz which reported this issue:

    intmatcher.cpp:1231:62: runtime error: division by zero
	    #0 0x6119d5 in IntegerMatcher::ApplyCNCorrection(float, int, int, int) tesseract/src/classify/intmatcher.cpp:1231:62
	    #1 0x5fe9c4 in tesseract::Classify::ComputeCorrectedRating(bool, int, double, double, int, int, int, int, int, unsigned char const*) tesseract/src/classify/adaptmatch.cpp:1213:29
	    #2 0x5fdc22 in tesseract::Classify::ExpandShapesAndApplyCorrections(ADAPT_CLASS_STRUCT**, bool, int, int, int, float, int, int, unsigned char const*, tesseract::UnicharRating*, ADAPT_RESULTS*) tesseract/src/classify/adaptmatch.cpp:1184:13
	    #3 0x5fe421 in tesseract::Classify::MasterMatcher(INT_TEMPLATES_STRUCT*, short, INT_FEATURE_STRUCT const*, unsigned char const*, ADAPT_CLASS_STRUCT**, int, int, TBOX const&, GenericVector<CP_RESULT_STRUCT> const&, ADAPT_RESULTS*) tesseract/src/classify/adaptmatch.cpp:1119:5
	    #4 0x6003eb in tesseract::Classify::CharNormTrainingSample(bool, int, tesseract::TrainingSample const&, GenericVector<tesseract::UnicharRating>*) tesseract/src/classify/adaptmatch.cpp:1374:5

See https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13712.

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

Author: stweil

src/classify/intmatcher.cpp

@@ -1229,7 +1229,7 @@ int IntegerMatcher::FindBestMatch(
 float IntegerMatcher::ApplyCNCorrection(float rating, int blob_length,
                                         int normalization_factor,
                                         int matcher_multiplier) {
-  return (rating * blob_length +
-          matcher_multiplier * normalization_factor / 256.0) /
-      (blob_length + matcher_multiplier);
+  int divisor = blob_length + matcher_multiplier;
+  return divisor == 0 ? 1.0f : (rating * blob_length +
+      matcher_multiplier * normalization_factor / 256.0f) / divisor;
 }