The issue comes up when I investigated issue #1641.
A func symbol defined in assembly code will be size of 0,
e.g., http://git.musl-libc.org/cgit/musl/tree/src/thread/x86_64/syscall_cp.s
symbol __cp_begin.
  .text
  .global __cp_begin
  .hidden __cp_begin
  ...
  __syscall_cp_asm:

  __cp_begin:
	mov (%rdi),%eax
	test %eax,%eax

and __cp_begin cannot be traced through bcc since
symbol resolution rejects any func symbol with size 0.

This patch removed size-must-not-zero restriction so that
the symbol like __cp_begin can be traced.
Command line: trace.py -p <pid> -U '<binary_path>:__cp_begin'
Signed-off-by: Yonghong Song <yhs@fb.com>