[minmax] Do nothing for empty suffix/prefix. (#202)

It's a semantical noop but slightly more efficient for the case when max/min is requested for an empty Collection.

Sources/Algorithms/MinMax.swift

@@ -256,14 +256,14 @@ extension Collection {
       """
     )
 
+    // Make sure we are within bounds.
+    let prefixCount = Swift.min(count, self.count)
+
     // Do nothing if we're prefixing nothing.
-    guard count > 0 else {
+    guard prefixCount > 0 else {
       return []
     }
 
-    // Make sure we are within bounds.
-    let prefixCount = Swift.min(count, self.count)
-
     // If we're attempting to prefix more than 10% of the collection, it's
     // faster to sort everything.
     guard prefixCount < (self.count / 10) else {
@@ -310,14 +310,14 @@ extension Collection {
       """
     )
 
+    // Make sure we are within bounds.
+    let suffixCount = Swift.min(count, self.count)
+
     // Do nothing if we're suffixing nothing.
-    guard count > 0 else {
+    guard suffixCount > 0 else {
       return []
     }
 
-    // Make sure we are within bounds.
-    let suffixCount = Swift.min(count, self.count)
-
     // If we're attempting to prefix more than 10% of the collection, it's
     // faster to sort everything.
     guard suffixCount < (self.count / 10) else {