zephyr: Add braces around single lines

The coding guidelines for mcuboot specify using braces around all
single-line constructs.  Add these to those lines missing this.
diff --git a/boot/zephyr/os.c b/boot/zephyr/os.c
index 3b910d5..0a5abbd 100644
--- a/boot/zephyr/os.c
+++ b/boot/zephyr/os.c
@@ -32,8 +32,9 @@
      * calls only come from within the app. */
     size_t total = nelem * size;
     void *buf = k_malloc(total);
-    if (buf)
+    if (buf) {
         memset(buf, 0, total);
+    }
     return buf;
 }