go-fuzz-build: set printer.SourcePos
go-fuzz-build: set printer.SourcePos

go-fuzz-build instruments code with coverage data.  This caused line
numbers in crash output to be wrong.  By setting printer.SourcePos in
the printer configuration we get `//line` directives added to the code,
which results in the line numbers from the original code.

Fixes #32.

dvyukov/go-fuzz,dvyukov/go-fuzz,octo/go-fuzz,octo/go-fuzz