checkpatch: improve get_quoted_string for TRACE_EVENT macros

The get_quoted_string function does not expect invalid arguments.

The $stat test can return non-statements for complicated macros like
TRACE_EVENT.

Allow the $stat block and test for vsprintf misuses to exceed the actual
block length and possibly test invalid lines by validating the arguments
of get_quoted_string.

Return "" if either get_quoted_string argument is undefined.

Miscellanea:

o Properly align the comment for the vsprintf extension test

Link: http://lkml.kernel.org/r/9e9725342ca3dfc0f5e3e0b8ca3c482b0e5712cc.1520356392.git.joe@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -1298,6 +1298,7 @@ sub sanitise_line {
 sub get_quoted_string {
 	my ($line, $rawline) = @_;
 
+	return "" if (!defined($line) || !defined($rawline));
 	return "" if ($line !~ m/($String)/g);
 	return substr($rawline, $-[0], $+[0] - $-[0]);
 }
@@ -5807,7 +5808,7 @@ sub process {
 			}
 		}
 
-		# check for vsprintf extension %p<foo> misuses
+# check for vsprintf extension %p<foo> misuses
 		if ($^V && $^V ge 5.10.0 &&
 		    defined $stat &&
 		    $stat =~ /^\+(?![^\{]*\{\s*).*\b(\w+)\s*\(.*$String\s*,/s &&