Update command/backtrace.sh

Co-authored-by: Joachim Ansorg <github@ja-dev.eu>

Author: rocky

command/backtrace.sh

@@ -169,7 +169,7 @@ function _Dbg_do_backtrace {
 	else
 	    lineno=${BASH_LINENO[$adjusted_pos-1]}
 	fi
-	filename=$(_Dbg_resolve_expand_filename "${BASH_SOURCE[$adjusted_pos]}")
+	filename="$(_Dbg_resolve_expand_filename "${BASH_SOURCE[$adjusted_pos]}")"
 	filename=$(_Dbg_adjust_filename "$filename")
 	_Dbg_msg "($_Dbg_parm_str) called from file \`$filename'" "at line $lineno"
 	if (( show_source )) ; then