fread.c: fix undefined behavior

A case of undefined behavior in __fread() has been detected by UBSAN
and reported by Mark Millard:

/usr/main-src/lib/libc/stdio/fread.c:133:10: runtime error: applying
zero offset to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior in
/usr/main-src/lib/libc/stdio/fread.c:133:10

While being benign (the NULL pointer is later passed to memcpy() with
a length argument of 0), this issue causes in the order of 600 Kyua
test cases to fail on systems running a world built with WITH_UBSAN
and WITH_ASAN.

The undefined behavior can be prevented by skipping operations that
have no effect for r == 0. Mark Millard has suggested to only skip
this code segment if fp->_p == NULL, but I have verified that for the
case of r == 0 no further argument checking is performed on the
addresses passed to memcpy() and thus no bugs are hidden from the
sanitizers due to the simpler condition chosen.

Reported by: Mark Millard (marklmi@yahoo.com)
Tested by: Mark Millard (marklmi@yahoo.com)
Differential Revision: https://reviews.freebsd.org/D33903
MFC after: 2 weeks