Allocate big enough buffer for text conversion

Buffer for conversion from Unicode to ASCII7 can
be small in some cases leading to invalid writes
which can result in crash.
This commit increases size of the buffer.

poppler/UTF.cc

@@ -446,7 +446,7 @@ void unicodeToAscii7(Unicode *in, int len, Unicode **ucs4_out,
     if (!in_idx)
       indices = nullptr;
     else
-      idx = (int *) gmallocn(len * 2 + 1, sizeof(int));
+      idx = (int *) gmallocn(len * 8 + 1, sizeof(int));
   }
 
   GooString gstr;