(docolon): Test (re_buffer.re_nsub > 0) rather than

(docolon): Test (re_buffer.re_nsub > 0) rather than
searching for `\(' to determine whether to return 0 or the empty
string.  Before it would improperly return '' if the pattern
contained a substring like this: `\\('.  From Karl Heuer.
For example, running expr c : '\\(' should print `0'.