fix invalid read in aligned_alloc

in case of mmap-obtained chunks, end points past the end of the
mapping and reading it may fault. since the value is not needed until
after the conditional, move the access to prevent invalid reads.

src/malloc/aligned_alloc.c

@@ -31,8 +31,6 @@ void *aligned_alloc(size_t align, size_t len)
 		return NULL;
 
 	header = ((size_t *)mem)[-1];
-	end = mem + (header & -8);
-	footer = ((size_t *)end)[-2];
 	new = (void *)((uintptr_t)mem + align-1 & -align);
 
 	if (!(header & 7)) {
@@ -41,6 +39,9 @@ void *aligned_alloc(size_t align, size_t len)
 		return new;
 	}
 
+	end = mem + (header & -8);
+	footer = ((size_t *)end)[-2];
+
 	((size_t *)mem)[-1] = header&7 | new-mem;
 	((size_t *)new)[-2] = footer&7 | new-mem;
 	((size_t *)new)[-1] = header&7 | end-new;