connection: Handle empty signature and signature with just a version.

Functions like wl_argument_from_va_list expect from get_next_argument,
to initialize details->type but when the signature is empty or contains
only version (like in desktop-shell-protocol.c in weston) it is left
uninitialized.

This patch fixes it, by initializing details->type with '\0' value,
signaling end of arguments.

Signed-off-by: Mariusz Ceier <mceier+wayland@gmail.com>

src/connection.c

@@ -419,6 +419,7 @@ get_next_argument(const char *signature, struct argument_details *details)
 			details->nullable = 1;
 		}
 	}
+	details->type = '\0';
 	return signature;
 }