util/nvmutil: reset errno on cmd_swap

If one of the checksums was valid, but the other was not,
errno would be set to E_CANCELED, but then the buffer would
be modified anyway; this is acceptable behaviour, and errno
would later be reset writing the GBE file, which is done
only on the condition that the buffer was modified, but
it's also a good idea to reset it here just in case.

This is not a bugfix, and no behavioural changes will be
observed by the user, but this may *prevent* a bug in the
future, so let's pre-fix that bug now.

util/nvmutil/nvmutil.c

@@ -345,6 +345,7 @@ cmd_swap(void)
 		gbeFileModified = 1;
 		nvmPartModified[0] = 1;
 		nvmPartModified[1] = 1;
+		errno = 0;
 	}
 }