* 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.
9fabffc880