play: Handle empty arguments to --key-map gracefully.

play: Handle empty arguments to --key-map gracefully.

Executing para_play with an empty argument to --key-map results in
a read which starts one byte past the allocated buffer:

	==24163== Invalid read of size 1
	==24163==    at 0x402A1DA: index (mc_replace_strmem.c:223)
	==24163==    by 0x804DA22: main (play.c:187)
	==24163==  Address 0x498e331 is 0 bytes after a block of size 1 alloc'd

This patch fixes the issue by rejecting empty arguments as invalid.