TCF Agent: Check if frame address is not zero for Step Over

Step functionality is missing the check for 0 in frame address.

Found on scenario:

1. Step Over on "jli_s" instruction (ARC) compares info->fp with ext->step_frame_fp.
2. Both are zero. Info was found on the upper frame.
3. "PC" was found and created Breakpoint for address too far (in the upper frame).
4. Target is resuming and we can hit breakpoint too far from interested function.

Having additional check will avoid potential problems.