check for mapped reader

[nclack]

Fixes an unchecked failure in acquire_map_read() caused when a caller fails to call acquire_unmap_read() at the right time.

For example:

acquire_map_read(runtime,0,&beg,&end);
// When end>beg
acquire_map_read(runtime,0,beg,end); // Should return AcquireStatus_Error

whereas

acquire_map_read(runtime,0,&beg,&end);
// When end>beg
acquire_unmap_read(runtime,0,&beg,&end);
acquire_map_read(runtime,0,beg,end); // Should return AcquireStatus_Ok

acquire-project/acquire-driver-common#34 depends on this

[aliddell]

I'm assuming unmapped reader (i.e., Channel_Expected_Unmapped_Reader) is the most common failure mode? The other option for status from channel_read_map is Channel_Error in the case of overflow. The error message here is misleading in that case.

[nclack]

fwiw, Channel_Error indicates something is wrong with the code in channel.c itself; it never happens due to a user error. That said, I'll improve the error checking here.

[andy-sweet]

Looks good to me.