Fix warning: ops.c: read_viminfo_register():  Dereference of null pointer: RI.

Problem    : Array access (via field 'y_array') results in a null pointer
             dereference @ 4487.
Diagnostic : Real issue.
Rationale  : If the array was previously freed and the size of the array
             (y_current->y_size) was not updated to zero, the loop @4486
             could be entered and a NULL pointer would be dereferenced.
Resolution : Use free_yank_all() to take care of the NULL check and
             to free the current yank register.
Merge pull request #1679 from oni-link/fix.clang.warning

Fix for clang analyzer warning in ops.c