grep: fix ASan heap-buffer-overflow.

Like the regular fgetln(), grep_fgetln() doesn't NUL-terminate the
string, which regexec() doesn't like. ASan just gained the ability to
intercept regexec(), which is why we didn't find this previously.

Bug: http://b/129089665
Test: adb shell grep -R /system -e "abc"
Test: toybox grep tests
Change-Id: Id707cea66a873b83bd763a3dcdf726ac7d062ce0