Fix issues with handling invalid regex in syntax files (zyedidia#2913)

* Fix panic due to invalid regex in a syntax file

When a user's custom syntax file has a malformed filename regex or
header regex, MakeHeaderYaml() returns error but we do not properly
handle it, which results in a panic due to a dereference of the `header`
pointer which is nil:

Micro encountered an error: runtime.errorString runtime error: invalid memory address or nil pointer dereference
runtime/panic.go:221 (0x44c367)
runtime/panic.go:220 (0x44c337)
github.com/zyedidia/micro/v2/internal/buffer/buffer.go:709 (0x82bc0f)
github.com/zyedidia/micro/v2/internal/buffer/buffer.go:392 (0x828292)
github.com/zyedidia/micro/v2/internal/buffer/buffer.go:261 (0x8278c8)
github.com/zyedidia/micro/v2/cmd/micro/micro.go:203 (0x8b9e7b)
github.com/zyedidia/micro/v2/cmd/micro/micro.go:331 (0x8ba9e5)
runtime/proc.go:255 (0x4386a7)
runtime/asm_amd64.s:1581 (0x467941)

* Do not ignore invalid filename regex error in a syntax file

When the filename regex in a syntax file is malformed but the subsequent
header regex is correct, the filename regex error gets silently ignored,
since the `err` value is overwritten by the subsequent successful header
regex result.

internal/buffer/buffer.go

@@ -699,6 +699,7 @@ func (b *Buffer) UpdateRules() {
 		header, err = highlight.MakeHeaderYaml(data)
 		if err != nil {
 			screen.TermMessage("Error parsing header for syntax file " + f.Name() + ": " + err.Error())
+			continue
 		}
 		file, err := highlight.ParseFile(data)
 		if err != nil {

pkg/highlight/parser.go

@@ -109,7 +109,7 @@ func MakeHeader(data []byte) (*Header, error) {
 	if fnameRgx != "" {
 		header.FtDetect[0], err = regexp.Compile(fnameRgx)
 	}
-	if headerRgx != "" {
+	if err == nil && headerRgx != "" {
 		header.FtDetect[1], err = regexp.Compile(headerRgx)
 	}
 
@@ -135,7 +135,7 @@ func MakeHeaderYaml(data []byte) (*Header, error) {
 	if hdrYaml.Detect.FNameRgx != "" {
 		header.FtDetect[0], err = regexp.Compile(hdrYaml.Detect.FNameRgx)
 	}
-	if hdrYaml.Detect.HeaderRgx != "" {
+	if err == nil && hdrYaml.Detect.HeaderRgx != "" {
 		header.FtDetect[1], err = regexp.Compile(hdrYaml.Detect.HeaderRgx)
 	}