checkpatch: check return of `git_commit_info`

Avoid string concatenation with an undefined variable when a reference to
a missing commit is contained in a `Fixes` tag.

Given this patch:

: From: Tamir Duberstein <[e-mail]>
: Subject: Test patch
: Date: Fri, 25 Oct 2024 19:30:51 -0400
:
: This is a test patch.
:
: Fixes: deadbeef111
: Signed-off-by: Tamir Duberstein <[e-mail]>
: --- /dev/null
: +++ b/new-file
: @@ -0,0 +1 @@
: +Test.

Before:

WARNING: Please use correct Fixes: style 'Fixes: <12 chars of sha1> ("<title line>")' - ie: 'Fixes: ("commit title")'
WARNING: Unknown commit id 'deadbeef111', maybe rebased or not pulled?
Use of uninitialized value $cid in concatenation (.) or string at scripts/checkpatch.pl line 3242.

After:

WARNING: Unknown commit id 'deadbeef111', maybe rebased or not pulled?

This patch also reduce duplication slightly.

[[e-mail]: s/12 chars of sha1/12+ chars of sha1/, per Jon]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Tamir Duberstein <[e-mail]>
Cc: Andy Whitcroft <[e-mail]>
Cc: Dwaipayan Ray <[e-mail]>
Cc: Joe Perches <[e-mail]>
Cc: Lukas Bulwahn <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

scripts/checkpatch.pl

···
 			my ($cid, $ctitle) = git_commit_info($orig_commit, $id,
 							     $title);
 
-			if ($ctitle ne $title || $tag_case || $tag_space ||
-			    $id_length || $id_case || !$title_has_quotes) {
+			if (defined($cid) && ($ctitle ne $title || $tag_case || $tag_space || $id_length || $id_case || !$title_has_quotes)) {
+				my $fixed = "Fixes: $cid (\"$ctitle\")";
 				if (WARN("BAD_FIXES_TAG",
-				     "Please use correct Fixes: style 'Fixes: <12 chars of sha1> (\"<title line>\")' - ie: 'Fixes: $cid (\"$ctitle\")'\n" . $herecurr) &&
+				     "Please use correct Fixes: style 'Fixes: <12+ chars of sha1> (\"<title line>\")' - ie: '$fixed'\n" . $herecurr) &&
 				    $fix) {
-					$fixed[$fixlinenr] = "Fixes: $cid (\"$ctitle\")";
+					$fixed[$fixlinenr] = $fixed;
 				}
 			}
 		}