mavlink fix for cause of intermittent crash

If the posix target is run and the rootfs is not created, then
there is an fopen in mavlink without a return value check and then a write
to the fd. When this condition occurs it tries to write to NULL and will
segfault.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/mavlink/mavlink_messages.cpp

@@ -409,9 +409,14 @@ protected:
 						snprintf(log_file_path, sizeof(log_file_path), PX4_ROOTFSDIR"/fs/microsd/%s", log_file_name);
 						fp = fopen(log_file_path, "ab");
 
-						/* write first message */
-						fputs(msg.text, fp);
-						fputs("\n", fp);
+						if (fp != NULL) {
+							/* write first message */
+							fputs(msg.text, fp);
+							fputs("\n", fp);
+						}
+						else {
+							warn("Failed to open %s errno=%d", log_file_path, errno);
+						}
 					}
 				}
 			}