Make sure double to int conversion is correct.

src/conversions.h

Index: src/conversions.h
diff --git a/src/conversions.h b/src/conversions.h
index 70559c9e9d7979f449110793bbdde122cd1d99a5..d62a172757daa09880b7e19f753b52e5ac3ed305 100644
--- a/src/conversions.h
+++ b/src/conversions.h
@@ -59,14 +59,23 @@ inline double SignedZero(bool negative) {
 
 // The fast double-to-(unsigned-)int conversion routine does not guarantee
 // rounding towards zero.
+// For NaN and values outside the int range, return INT_MIN or INT_MAX.
+inline int FastD2IChecked(double x) {
+  if (!(x >= INT_MIN)) return INT_MIN;  // Negation to catch NaNs.
+  if (x > INT_MAX) return INT_MAX;
+  return static_cast<int>(x);
+}
+
+
+// The fast double-to-(unsigned-)int conversion routine does not guarantee
+// rounding towards zero.
 // The result is unspecified if x is infinite or NaN, or if the rounded
 // integer value is outside the range of type int.
 inline int FastD2I(double x) {
-  // The static_cast convertion from double to int used to be slow, but
-  // as new benchmarks show, now it is much faster than lrint().
   return static_cast<int>(x);
 }
 
+
 inline unsigned int FastD2UI(double x);