Avoid panics when decode hooks receive named strings

[a3ylf]

Built-in string decode hooks accept any source whose reflected kind is string, but asserting data.(string) panics for defined types such as type ConfigValue string. Read the reflected string value after the existing kind checks so these inputs follow the same parsers and error paths as ordinary strings.

Tests exercise named and ordinary strings across the built-in conversions, plus full decoding into named-string slices and IP networks. go test -race -shuffle=on ./... passes. The changed code passes lint; the full lint run with v2.13.2 reports three existing govet findings in unchanged mapstructure.go.