FST: Fix undefinied behaviour due to order of evaluation of the method

parameters which was leading to files not having the checksum value set.

Author: esindril

fst/XrdFstOfsFile.cc

@@ -3442,8 +3442,9 @@ XrdFstOfsFile::VerifyChecksum()
                     "fxid=%08llx errno=%d", mFileId, errno);
           }
 
-          if (io->attrSet("user.eos.checksum", mCheckSum->GetBinChecksum(checksumlen),
-                          checksumlen)) {
+          const char* ptr = mCheckSum->GetBinChecksum(checksumlen);
+
+          if (io->attrSet("user.eos.checksum", ptr, checksumlen)) {
             eos_err("msg=\"unable to set extended attr <eos.checksum> \" "
                     "fxid=%08llx errno=%d", mFileId, errno);
           }

fst/storage/Verify.cc

@@ -241,8 +241,8 @@ Storage::Verify()
 
           // Update the extended attributes
           if (io) {
-            (void)io->attrSet("user.eos.checksum", checksummer->GetBinChecksum(checksumlen),
-                              checksumlen);
+            const char* ptr = checksummer->GetBinChecksum(checksumlen);
+            (void)io->attrSet("user.eos.checksum", ptr, checksumlen);
             (void)io->attrSet("user.eos.checksumtype", checksummer->GetName(),
                               strlen(checksummer->GetName()));
             (void)io->attrSet("user.eos.filecxerror", "0", 1);