[woff2] Fix segfault.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=23402

* src/sfnt/sfwoff2.c (get_x_mins): Check whether `loca' table
exists.

ChangeLog

@@ -1,3 +1,14 @@
+2020-06-19  Werner Lemberg  <wl@gnu.org>
+
+	[woff2] Fix segfault.
+
+	Reported as
+
+	  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=23402
+
+	* src/sfnt/sfwoff2.c (get_x_mins): Check whether `loca' table
+	exists.
+
 2020-06-19  Stephen McDowell  <svenevs.dev@gmail.com>
 
 	[sfnt] Support Intel compilers.

src/sfnt/sfwoff2.c

@@ -1286,6 +1286,12 @@
       return FT_THROW( Invalid_Table );
     }
 
+    if ( !info->loca_table )
+    {
+      FT_ERROR(( "`loca' table is missing.\n" ));
+      return FT_THROW( Invalid_Table );
+    }
+
     /* Read `numGlyphs' field from `maxp' table. */
     if ( FT_STREAM_SEEK( maxp_table->src_offset ) || FT_STREAM_SKIP( 8 ) )
       return error;