[tools] testbot/TestWTBS: The prefix may be undefined in GrepFile().

[Francois Gouget]

Signed-off-by: Francois Gouget <fgouget(a)codeweavers.com>
---
 testbot/tests/TestWTBS | 1 +
 1 file changed, 1 insertion(+)

diff --git a/testbot/tests/TestWTBS b/testbot/tests/TestWTBS
index 48376649e..dbd1130ca 100755
--- a/testbot/tests/TestWTBS
+++ b/testbot/tests/TestWTBS
@@ -757,6 +757,7 @@ sub GrepFile($$$$;$)
 {
   my ($Key, $Dir, $FileName, $GrepInfo, $GrepPrefix) = @_;
 
+  $GrepPrefix ||= "";
   my @Grep = @{$GrepInfo->{"${GrepPrefix}Grep"} || []};
   my @GrepV = @{$GrepInfo->{"${GrepPrefix}GrepV"} || []};
   return if (!@Grep and !@GrepV);
-- 
2.20.1