Fix UBSan warning in arithmetic decoder

Very similar to the ones that were fixed in the Huffman decoders in
8e9cef2.  These are innocuous.

Refer to https://bugzilla.mozilla.org/show_bug.cgi?id=1304567.

jdarith.c

@@ -21,6 +21,9 @@
 #include "jpeglib.h"
 
 
+#define NEG_1 ((unsigned int)-1)
+
+
 /* Expanded entropy decoder object for arithmetic decoding. */
 
 typedef struct {
@@ -450,7 +453,7 @@ decode_mcu_AC_refine (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
   tbl = cinfo->cur_comp_info[0]->ac_tbl_no;
 
   p1 = 1 << cinfo->Al;          /* 1 in the bit position being coded */
-  m1 = (-1) << cinfo->Al;       /* -1 in the bit position being coded */
+  m1 = (NEG_1) << cinfo->Al;    /* -1 in the bit position being coded */
 
   /* Establish EOBx (previous stage end-of-block) index */
   for (kex = cinfo->Se; kex > 0; kex--)