Fix elf_gnu_ifunc_resolve_by_got buglet

The next patch will add a call to elf_gnu_ifunc_resolve_by_got that
trips on a latent buglet -- the function is writing to its output
parameter even if the address wasn't found, confusing the caller.  The
function's intro comment says:

  /* Try to find the target resolved function entry address of a STT_GNU_IFUNC
     function NAME.  If the address is found it is stored to *ADDR_P (if ADDR_P
     is not NULL) and the function returns 1.  It returns 0 otherwise.

So fix the function accordingly.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* elfread.c (elf_gnu_ifunc_resolve_by_got): Don't write to *ADDR_P
	unless we actually resolved the ifunc.