[lzw] Avoid buffer overrun.

Reported as

  https://bugzilla.mozilla.org/show_bug.cgi?id=1273283

* src/lzw/ftzopen.c (ft_lzwstate_refill): Ensure `buf_size' doesn't
underflow.

ChangeLog

@@ -1,3 +1,14 @@
+2016-08-16  Werner Lemberg  <wl@gnu.org>
+
+	[lzw] Avoid buffer overrun.
+
+	Reported as
+
+	  https://bugzilla.mozilla.org/show_bug.cgi?id=1273283
+
+	* src/lzw/ftzopen.c (ft_lzwstate_refill): Ensure `buf_size' doesn't
+	underflow.
+
 2016-08-16  Werner Lemberg  <wl@gnu.org>
 
 	[truetype] Fix compiler warning.

src/lzw/ftzopen.c

@@ -42,7 +42,12 @@
     state->buf_total += count;
     state->in_eof     = FT_BOOL( count < state->num_bits );
     state->buf_offset = 0;
-    state->buf_size   = ( state->buf_size << 3 ) - ( state->num_bits - 1 );
+
+    state->buf_size <<= 3;
+    if ( state->buf_size > state->num_bits )
+      state->buf_size -= state->num_bits - 1;
+    else
+      return -1; /* not enough data */
 
     if ( count == 0 )  /* end of file */
       return -1;