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:
2018-04-26  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.

1 files changed, 5 insertions, 3 deletions

diff --git a/gdb/elfread.c b/gdb/elfread.c
index 16a692d..42a2c92 100644
--- a/gdb/elfread.c
+++ b/gdb/elfread.c
@@ -833,10 +833,12 @@ elf_gnu_ifunc_resolve_by_got (const char *name, CORE_ADDR *addr_p)
 						 &current_target);
       addr = gdbarch_addr_bits_remove (gdbarch, addr);
 
-      if (addr_p)
-	*addr_p = addr;
       if (elf_gnu_ifunc_record_cache (name, addr))
-	return 1;
+	{
+	  if (addr_p != NULL)
+	    *addr_p = addr;
+	  return 1;
+	}
     }
 
   return 0;