patch 8.2.3612: using freed memory with regexp using a mark

Problem:    Using freed memory with regexp using a mark.
Solution:   Get the line again after getting the mark position.

src/regexp.c

@@ -1129,7 +1129,7 @@ typedef struct {
     // The current match-position is stord in these variables:
     linenr_T	lnum;		// line number, relative to first line
     char_u	*line;		// start of current line
-    char_u	*input;		// current input, points into "regline"
+    char_u	*input;		// current input, points into "line"
 
     int	need_clear_subexpr;	// subexpressions still need to be cleared
 #ifdef FEAT_SYN_HL