e2fsprogs: fix potential null ptr defef in check_for_modules()

e2fsprogs: fix potential null ptr defef in check_for_modules()

The coverity scanner found this one.

If a line in modules.dep has a ":" but no "/" then:

                if ((cp = strchr(buf, ':')) != NULL)
                        *cp = 0;
                else
                        continue;
                if ((cp = strrchr(buf, '/')) != NULL)
                        cp++;
                /* XXX else cp is still null */
                i = strlen(cp);

... we will deref a null pointer (cp).  This can be
demonstrated by putting a line like:

foo.ko:

into modules.dep.  The below change just says that if no "/" is
found, treat the whole string as the module name.

Addresses-Red-Hat-Bugzilla: #486997

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>