leptonica.py: Fix exception on certain types of barcode failures

Closes #322

docs/release_notes.rst

@@ -17,7 +17,9 @@ Note that it is licensed under GPLv3, so scripts that ``import ocrmypdf`` and ar
 v7.4.1
 ------
 
--   Fix a few documentation problems.
+-   Fixed a few documentation problems.
+
+-   Fixed an unhandled exception when attempting to mask barcodes. #322 <https://github.com/jbarlow83/OCRmyPDF/issues/322>`_
 
 v7.4.0
 ------

src/ocrmypdf/leptonica.py

@@ -567,8 +567,8 @@ class Pix(LeptonicaObject):
             with _LeptonicaErrorTrap():
                 pix = Pix(lept.pixConvertTo8(self._cdata, 0))
                 pixa_candidates = PixArray(lept.pixExtractBarcodes(pix._cdata, 0))
-                with suppress(FileNotFoundError):
-                    os.unlink('junkpixt.png')  # leptonica may produce this
+                if not pixa_candidates:
+                    return
                 sarray = StringArray(lept.pixReadBarcodes(
                     pixa_candidates._cdata,
                     lept.L_BF_ANY,
@@ -576,8 +576,12 @@ class Pix(LeptonicaObject):
                     ffi.NULL,
                     0
                 ))
-        except (LeptonicaError, ValueError) as e:
+        except (LeptonicaError, ValueError, IndexError) as e:
             return
+        finally:
+            with suppress(FileNotFoundError):
+                os.unlink('junkpixt.png')  # leptonica may produce this
+                os.unlink('junkpixt')
 
         for n, s in enumerate(sarray):
             decoded = s.decode()