[letterspace] fix crash in kerning by keeping the default on failed l…

…ookup

With commit 8c0dd0e kern accesses were fixed but the results never
tested for the lookup, which caused the default which is set up before
to be nil’ed. Fix this by reusing the value we initialized the kern
factor to.

src/luaotfload-letterspace.lua

@@ -413,7 +413,7 @@ kerncharacters = function (head)
                 else
                   local kern = 0
                   local kerns = prevchardata.kerns
-                  if kerns then kern = kerns[lastchar] end
+                  if kerns then kern = kerns[lastchar] or kern end
                   krn = kern + quaddata[lastfont]*krn -- here
                   insert_node_before(head,start,kern_injector(fillup,krn))
                 end
@@ -488,7 +488,7 @@ kerncharacters = function (head)
                   --- font doesn’t contain the glyph
                 else
                   local kerns = prevchardata.kerns
-                  if kerns then kern = kerns[lastchar] end
+                  if kerns then kern = kerns[lastchar] or kern end
                 end
               end
               krn = kern + quaddata[lastfont]*krn -- here