BUILD: ssl_ckch: fix "maybe-uninitialized" build error on gcc-9.4 + ARM

As reported in issue #1755, gcc-9.3 and 9.4 emit a "maybe-uninitialized"
warning in cli_io_handler_commit_cafile_crlfile() because it sees that
when the "path" variable is not set, we're jumping to the error label
inside the loop but cannot see that the new state will avoid the places
where the value is used. Thus it's a false positive but a difficult one.
Let's just preset the value to NULL to make it happy.

This was introduced in 2.7-dev by commit ddc8e1cf8 ("MINOR: ssl_ckch:
Simplify I/O handler to commit changes on CA/CRL entry"), thus no
backport is needed for now.

src/ssl_ckch.c

@@ -2710,6 +2710,7 @@ static int cli_io_handler_commit_cafile_crlfile(struct appctx *appctx)
 			path = crlfile_transaction.path;
 			break;
 		default:
+			path = NULL;
 			goto error;
 	}