Avoid memory leak in rmtree() when path cannot be opened

An allocation done for the directory names to recurse into for their
deletion was done before OPENDIR(), so, assuming that a failure happens,
this could leak a bit of memory.

Author: Ranier Vilela
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/CAEudQAoN3-2ZKBALThnEk_q2hu8En5A0WG9O+5siJTQKVZzoWQ@mail.gmail.com

src/common/rmtree.c

@@ -55,7 +55,7 @@ rmtree(const char *path, bool rmtopdir)
 	bool		result = true;
 	size_t		dirnames_size = 0;
 	size_t		dirnames_capacity = 8;
-	char	  **dirnames = palloc(sizeof(char *) * dirnames_capacity);
+	char	  **dirnames;
 
 	dir = OPENDIR(path);
 	if (dir == NULL)
@@ -64,6 +64,8 @@ rmtree(const char *path, bool rmtopdir)
 		return false;
 	}
 
+	dirnames = (char **) palloc(sizeof(char *) * dirnames_capacity);
+
 	while (errno = 0, (de = readdir(dir)))
 	{
 		if (strcmp(de->d_name, ".") == 0 ||