Fixes trying to close an invalid flash map handle

This error was catched by Coverity and it happens when a fail occurs
opening a flash map handle, which is not checked by the close
routine.

Right now this only affects Zephyr, but extra checking was added
assuming that in a future Mynewt implementation close could actually
be changed to do something.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 2b0002f..ca78c6e 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -745,8 +745,12 @@
     rc = 0;
 
 done:
-    flash_area_close(fap_src);
-    flash_area_close(fap_dst);
+    if (fap_src) {
+        flash_area_close(fap_src);
+    }
+    if (fap_dst) {
+        flash_area_close(fap_dst);
+    }
     return rc;
 }