Fix capturing exit status to properly display libtar results

Error conditions were always showing successful in the TWRP GUI
making it much more difficult to determine if a backup or restore
was actually successful.
diff --git a/twrpTar.cpp b/twrpTar.cpp
index 159c1d1..cf225db 100644
--- a/twrpTar.cpp
+++ b/twrpTar.cpp
@@ -107,7 +107,7 @@
 				LOGI("Child process ended with signal: %d\n", WTERMSIG(status));
 				return -1;
 			}
-			else if (WIFEXITED(status) != 0)
+			else if (WEXITSTATUS(status) == 0)
 				LOGI("Tar creation successful\n");
 			else {
 				LOGI("Tar creation failed\n");
@@ -141,7 +141,7 @@
 				LOGI("Child process ended with signal: %d\n", WTERMSIG(status));
 				return -1;
 			}
-			else if (WIFEXITED(status) != 0)
+			else if (WEXITSTATUS(status) == 0)
 				LOGI("Tar extraction successful\n");
 			else {
 				LOGI("Tar extraction failed\n");