Fix intset midpoint selection

The classic (min+max)/2 is provably unsafe.  Fixed
as recommended in research:
http://googleresearch.blogspot.com/2006/06/extra-extra-read-all-about-it-nearly.html

Fix inspired by @wjin, but I used a different approach.
(later, I found @kuebler fixed the same issue too).

Fixes #1741, #1602

src/intset.c

@@ -133,7 +133,7 @@ static uint8_t intsetSearch(intset *is, int64_t value, uint32_t *pos) {
     }
 
     while(max >= min) {
-        mid = (min+max)/2;
+        mid = ((unsigned int)min + (unsigned int)max) >> 1;
         cur = _intsetGet(is,mid);
         if (value > cur) {
             min = mid+1;