bug#17556: 24.4.50; fix math-bignum

[Leo Liu]

Due to integer overflow (math-bignum most-negative-fixnum) is incorrect.
Any objection if I fix it like in this patch:

=== modified file 'lisp/calc/calc.el'
--- lisp/calc/calc.el   2014-01-01 07:43:34 +0000
+++ lisp/calc/calc.el   2014-05-23 04:42:18 +0000
@@ -2773,9 +2773,15 @@
 
 ;; Coerce integer A to be a bignum.  [B S]
 (defun math-bignum (a)
-  (if (>= a 0)
-      (cons 'bigpos (math-bignum-big a))
-    (cons 'bigneg (math-bignum-big (- a)))))
+  (cond
+   ((>= a 0)
+    (cons 'bigpos (math-bignum-big a)))
+   ((= a most-negative-fixnum)
+    ;; Note: (- most-negative-fixnum) is most-negative-fixnum
+    (math-sub (cons 'bigneg (math-bignum-big (- (1+ a))))
+             1))
+   (t
+    (cons 'bigneg (math-bignum-big (- a))))))
 
 (defun math-bignum-big (a)   ; [L s]
   (if (= a 0)