patch 8.2.5046: vim_regsub() can overwrite the destination

Problem:    vim_regsub() can overwrite the destination.
Solution:   Pass the destination length, give an error when it doesn't fit.

src/regexp.h

@@ -177,4 +177,9 @@ struct regengine
     //char_u	*expr;
 };
 
+// Flags used by vim_regsub() and vim_regsub_both()
+#define REGSUB_COPY	    1
+#define REGSUB_MAGIC	    2
+#define REGSUB_BACKSLASH    4
+
 #endif	// _REGEXP_H