Fix 'adb backup' command line validation

The host side wasn't properly checking for argument-list sufficiency
*after* removing any [-f filename] sequence.

Fixes bug 5164135

Change-Id: I7bc49e37ef168182088e0e664b6897dd2a088ebf

adb/commandline.c

@@ -579,9 +579,6 @@ static int backup(int argc, char** argv) {
     int fd, outFd;
     int i, j;
 
-    /* bare "adb backup" is not a valid command */
-    if (argc < 2) return usage();
-
     /* find, extract, and use any -f argument */
     for (i = 1; i < argc; i++) {
         if (!strcmp("-f", argv[i])) {
@@ -598,6 +595,9 @@ static int backup(int argc, char** argv) {
         }
     }
 
+    /* bare "adb backup" or "adb backup -f filename" are not valid invocations */
+    if (argc < 2) return usage();
+
     outFd = adb_open_mode(filename, O_WRONLY | O_CREAT | O_TRUNC, 0640);
     if (outFd < 0) {
         fprintf(stderr, "adb: unable to open file %s\n", filename);