imgact_elf: Unconditionally initialize a variable in a note handler

In the sb == NULL case, we are computing the size of the note using a
dummy sbuf drain handler which counts bytes and discards the contents of
the buffer, so the fact that "structsize" is uninitialized doesn't
matter.  But, the compiler may complain about this, so we might as well
just initialize it unconditionally to silence the warning, as other
handlers already do.

PR:		292811
MFC after:	1 week

sys/kern/imgact_elf.c

@@ -2714,6 +2714,7 @@ __elfN(note_procstat_auxv)(void *arg, struct sbuf *sb, size_t *sizep)
 	int structsize;
 
 	p = arg;
+	structsize = sizeof(Elf_Auxinfo);
 	if (sb == NULL) {
 		size = 0;
 		sb = sbuf_new(NULL, NULL, AT_COUNT * sizeof(Elf_Auxinfo),
@@ -2727,7 +2728,6 @@ __elfN(note_procstat_auxv)(void *arg, struct sbuf *sb, size_t *sizep)
 		sbuf_delete(sb);
 		*sizep = size;
 	} else {
-		structsize = sizeof(Elf_Auxinfo);
 		sbuf_bcat(sb, &structsize, sizeof(structsize));
 		PHOLD(p);
 		proc_getauxv(curthread, p, sb);