grep: fail if call could output and name is null

grep: fail if call could output and name is null

grep_source(), which performs much of the work for Git's grep library,
allows passing an arbitrary struct grep_source which represents the text
which grep_source() should search to match a pattern in the provided
struct grep_opt. In most callers, the grep_source::name field is set to
an appropriate prefix to print before a colon when a result matches:

	README:Git is an Open Source project covered by the GNU General

One caller, grep_buffer(), leaves the grep_source::name field set to
NULL because there isn't enough context to determine an appropriate name
for this kind of output line. In practice, this has been fine: the only
caller of grep_buffer() is "git log --grep", and that caller sets
grep_opt::status_only, which disables output and only checks whether a
match exists. But this is brittle: a future caller can call
grep_buffer() without grep_opt::status_only set, and as soon as it hits
a match, grep_source() will try to print the match and segfault:

	(null):Git is an Open Source project covered by the GNU General

For example, a future caller might want to print all matching lines from
commits which match a regex.

Futureproof by diagnosing early a use of the API that could trigger that
condition, before we know whether the pattern matches:

	BUG: grep.c:1783: grep call which could print a name requires
		grep_source.name be non-NULL
	Aborted

This way, the caller's author gets an indication of how to fix the issue
- by providing grep_source::name or setting grep_opt::status_only - and
they are warned of the potential for a segfault unconditionally, rather
than only if there is a match.

Noticed while adding such a call to a tutorial on revision walks.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>