coregrind/m_debuginfo: don't try to examine zero sized mmapped files

coregrind/m_debuginfo: don't try to examine zero sized mmapped files

When run on an nfs filesystem memcheck/tests/pointer-trace fails
because it generates warnings "connection to image failed". This is
caused by trying to mmap a deleted file which the nfs file system
represents as a (hidden) regular file. This is normally not a problem
except when that file is empty.

Fix this by not trying to check whether a file is an ELF or MACHO
against an empty (regular) file in di_notify_mmap. An empty file is
never a valid ELF or MACHO file (and cannot be represented as
DiImage).

https://bugs.kde.org/show_bug.cgi?id=501119