cmd/pprof: fix EOF handling when getting function source

getFunctionSource gathers five lines of "margin" around every
requested sample line.  However, if this margin went past the
end of the source file, getFunctionSource would encounter an
io.EOF error and abort with this error, resulting in listings
like

    (pprof) list main.main
    ROUTINE ======================== main.main in ...
    0      8.33s (flat, cum) 99.17% of Total
    Error: EOF
    (pprof)

Modify the error handling in getFunctionSource so io.EOF is
always considered non-fatal.  If it reaches EOF, it simply
returns the lines it has.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/172600043