pango_font_info: Fix runtime error messages from Pango
pango_font_info: Fix runtime error messages from Pango

pango_coverage_get and pango_coverage_unref should not be called
with coverage == nullptr.

pango_font_get_coverage should not be called with font == nullptr.

Otherwise Pango prints runtime error messages:

    (process:12657): Pango-CRITICAL **: pango_coverage_get: assertion 'coverage != NULL' failed
    (process:12657): Pango-CRITICAL **: pango_coverage_unref: assertion 'coverage != NULL' failed
    (process:12657): Pango-CRITICAL **: pango_font_get_coverage: assertion 'font != NULL' failed
    (process:12657): GLib-GObject-CRITICAL **: g_object_unref: assertion 'G_IS_OBJECT (object)' failed

Typically those errors occur if a required font is not installed,
so this can be a quite common error.

Fix also a potential resource leak in PangoFontInfo::CoversUTF8Text.

Signed-off-by: Stefan Weil <8d4c780fcfdc41841e5070f4c43da8958ba6aec0@weilnetz.de>

stweil/tesseract,UB-Mannheim/tesseract,tesseract-ocr/tesseract,UB-Mannheim/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,stweil/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,stweil/tesseract,UB-Mannheim/tesseract,tesseract-ocr/tesseract,jbarlow83/tesseract,jbarlow83/tesseract,jbarlow83/tesseract,amitdo/tesseract,stweil/tesseract,UB-Mannheim/tesseract,amitdo/tesseract,stweil/tesseract,amitdo/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,jbarlow83/tesseract,UB-Mannheim/tesseract