Fix Time#add_span boundary check

Previously, this method did not handle times at the min or max range with positive or
negative offsets correctly because `@seconds` can legitimately be `< 0` or `> MAX_SECONDS`
when the offset is taken into account.

spec/std/time/time_spec.cr

@@ -121,6 +121,37 @@ describe Time do
     end
   end
 
+  it "#add_span checks boundary at time min" do
+    {5 * 3600, 1, 0, -1, -5 * 3600}.each do |offset|
+      location = Time::Location.fixed(offset)
+
+      time = Time.new(1, 1, 1, location: location)
+      time.add_span(0, 1).should eq Time.new(1, 1, 1, nanosecond: 1, location: location)
+      time.add_span(0, 0).should eq time
+      expect_raises(ArgumentError) do
+        time.add_span(0, -1)
+      end
+    end
+  end
+
+  it "#add_span checks boundary at time max" do
+    {5 * 3600, 1, 0, -1, -5 * 3600}.each do |offset|
+      location = Time::Location.fixed(offset)
+
+      time = Time.new(9999, 12, 31, 23, 59, 59, nanosecond: 999_999_999, location: location)
+      time.add_span(0, -1).should eq Time.new(9999, 12, 31, 23, 59, 59, nanosecond: 999_999_998, location: location)
+      time.add_span(0, 0).should eq time
+      expect_raises(ArgumentError) do
+        time.add_span(0, 1)
+      end
+    end
+  end
+
+  it "adds zero span" do
+    time = Time.now
+    time.add_span(0, 0).should eq time
+  end
+
   it "add days" do
     t1 = Time.utc(2002, 2, 25, 15, 25, 13)
     t1 = t1 + 3.days

src/time.cr

@@ -297,7 +297,7 @@ struct Time
       nanoseconds += NANOSECONDS_PER_SECOND
     end
 
-    unless 0 <= seconds <= MAX_SECONDS
+    unless 0 <= seconds + offset <= MAX_SECONDS
       raise ArgumentError.new "Invalid time"
     end