Don't include undef in cross reference table.

From upstream's 55f7fb8848.

Change-Id: I54ebc182addbf643bebc78aab03ba1327e24e2e7
Reviewed-on: https://boringssl-review.googlesource.com/2470
Reviewed-by: Adam Langley <agl@google.com>

crypto/obj/obj_xref.h

@@ -53,9 +53,6 @@ static const nid_triple sigoid_srt[] =
 
 static const nid_triple * const sigoid_srt_xref[] =
 	{
-	&sigoid_srt[29],
-	&sigoid_srt[17],
-	&sigoid_srt[18],
 	&sigoid_srt[0],
 	&sigoid_srt[1],
 	&sigoid_srt[7],

crypto/obj/obj_xref.pl

@@ -90,7 +90,10 @@ EOF
 
 foreach (@srt2)
 	{
-	my $x = $xref_tbl{$_}[2];
+	my ($p1, $p2, $x) = @{$xref_tbl{$_}};
+	# If digest or signature algorithm is "undef" then the algorithm
+	# needs special handling and is excluded from the cross reference table.
+	next if $p1 eq "undef" || $p2 eq "undef";
 	print "\t\&sigoid_srt\[$x\],\n";
 	}