Fix insufficient slice check (gin-gonic#2755)

(cherry picked from commit 1d0f938)

tree.go

@@ -472,7 +472,7 @@ walk: // Outer loop for walking the tree
 					}
 
 					// Save param value
-					if params != nil {
+					if params != nil && cap(*params) > 0 {
 						if value.params == nil {
 							value.params = params
 						}

tree_test.go

@@ -836,6 +836,19 @@ func TestTreeInvalidNodeType(t *testing.T) {
 	}
 }
 
+func TestTreeInvalidParamsType(t *testing.T) {
+	tree := &node{}
+	tree.wildChild = true
+	tree.children = append(tree.children, &node{})
+	tree.children[0].nType = 2
+
+	// set invalid Params type
+	params := make(Params, 0, 0)
+
+	// try to trigger slice bounds out of range with capacity 0
+	tree.getValue("/test", &params, false)
+}
+
 func TestTreeWildcardConflictEx(t *testing.T) {
 	conflicts := [...]struct {
 		route        string