patch 9.2.0056: memory leak in ex_substitute

Problem:  memory leak in ex_substitute
Solution: Free the variable (Huihui Huang)

fixes:  #19501
closes: #19506

Signed-off-by: Huihui Huang <625173@qq.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/ex_cmds.c

@@ -4130,8 +4130,11 @@ ex_substitute(exarg_T *eap)
 		vim_free(old_sub);
 		old_sub = vim_strsave(sub);
 		if (old_sub == NULL)
+		{
 		    // out of memory
+		    vim_free(sub);
 		    return;
+		}
 	    }
 	}
     }

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    56,
 /**/
     55,
 /**/