hrtime.h: fix typo in non-builtin overflow check

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64592 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hrtime.h

@@ -64,7 +64,7 @@ rb_hrtime_mul(rb_hrtime_t a, rb_hrtime_t b)
     if (__builtin_mul_overflow(a, b, &c))
         return RB_HRTIME_MAX;
 #else
-    if (b != 0 && b > RB_HRTIME_MAX / b) /* overflow */
+    if (b != 0 && a > RB_HRTIME_MAX / b) /* overflow */
         return RB_HRTIME_MAX;
     c = a * b;
 #endif