bug#18266: grep -P and invalid exits with error

[Paul Eggert]

Thanks, but that patch seems to depend on libpcre internals, in that it 
"knows" that pcre_exec cannot possibly succeed without first checking 
its entire input buffer for invalid UTF-8 bytes.  Even if that's true 
now, it reflects a performance bug that might be fixed in a future 
libpcre version.

Also, I don't see why grep needs to copy the buffer when there's an 
encoding error.  Why not simply rerun the matcher on the initial prefix 
that doesn't have an encoding-error byte, and then (if that doesn't find 
a match), try matching the suffix after the encoding-error byte?  This 
approach would not only avoid the buffer copy, it would avoid knowledge 
of libpcre internals.