ofz#3782 libjpeg ubsan woes

https://github.com/libjpeg-turbo/libjpeg-turbo/issues/171

Change-Id: Ic6d7e27f88a542d78f3257418099c8fd875b04c9
Reviewed-on: https://gerrit.libreoffice.org/43812Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

external/libjpeg-turbo/ubsan.patch

@@ -17,3 +17,23 @@
      put_bits -= 8;
    }
  
+--- jdarith.c
++++ jdarith.c
+@@ -306,7 +306,7 @@
+       while (m >>= 1)
+         if (arith_decode(cinfo, st)) v |= m;
+       v += 1; if (sign) v = -v;
+-      entropy->last_dc_val[ci] += v;
++      entropy->last_dc_val[ci] = (entropy->last_dc_val[ci] + v) & 0xffff;
+     }
+ 
+     /* Scale and output the DC coefficient (assumes jpeg_natural_order[0]=0) */
+@@ -564,7 +564,7 @@
+       while (m >>= 1)
+         if (arith_decode(cinfo, st)) v |= m;
+       v += 1; if (sign) v = -v;
+-      entropy->last_dc_val[ci] += v;
++      entropy->last_dc_val[ci] = (entropy->last_dc_val[ci] + v) & 0xffff;
+     }
+ 
+     if (block)