Fix panic with substr interpolation function by invalid offset

Fixes #17041

config/interpolate_funcs.go

@@ -1577,7 +1577,7 @@ func interpolationFuncSubstr() ast.Function {
 				return nil, fmt.Errorf("length should be a non-negative integer")
 			}
 
-			if offset > len(str) {
+			if offset > len(str) || offset < 0 {
 				return nil, fmt.Errorf("offset cannot be larger than the length of the string")
 			}
 

config/interpolate_funcs_test.go

@@ -2568,6 +2568,11 @@ func TestInterpolateFuncSubstr(t *testing.T) {
 				nil,
 				true,
 			},
+			{
+				`${substr("foo", -4, -1)}`,
+				nil,
+				true,
+			},
 
 			// invalid length
 			{