[SVG -> OTF Converter] Crashes when SVG font is invalid

[SVG -> OTF Converter] Crashes when SVG font is invalid
https://bugs.webkit.org/show_bug.cgi?id=140378

Reviewed by Antti Koivisto.

Because CachedSVGFonts are cached, they have to be able to be used
in subsequent documents regardless how the first document left it.

Tests: fast/css/font-face-svg-decoding-error.html
       svg/custom/svg-fonts-in-html.html
       svg/text/text-overflow-ellipsis-svgfont-kerning-ligatures.html

* loader/cache/CachedFont.cpp:
(WebCore::CachedFont::ensureCustomFontData):
* loader/cache/CachedFont.h:
* loader/cache/CachedSVGFont.cpp:
(WebCore::CachedSVGFont::getFontData):
(WebCore::CachedSVGFont::ensureCustomFontData):
(WebCore::CachedSVGFont::maybeInitializeExternalSVGFontElement):
* loader/cache/CachedSVGFont.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@178628 268f45cc-cd09-0410-ab3c-d52691b4dbfc