Move "fastboot oem" to std::string.

Not sure how this code got missed when we moved everything else off
C string handling...

  $ adb reboot bootloader
  $ fastboot oem `perl -e 'print "x"x1024;'`

Before:
  <crashes>

After:
  error: Command length (1028) exceeds maximum size (64)

(The error says 1028 instead of 1024 because it includes the "oem ".)

Bug: http://b/36232671
Test: fastboot oem `perl -e 'print "x"x1024;'`
Change-Id: Ib4664e49222bd2b71be5aa3fe81f386d6073414f
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index a4a0b52..2927b16 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -1245,20 +1245,17 @@
     return 0;
 }
 
-static int do_oem_command(int argc, char **argv)
-{
-    char command[256];
+static int do_oem_command(int argc, char** argv) {
     if (argc <= 1) return 0;
 
-    command[0] = 0;
-    while(1) {
-        strcat(command,*argv);
+    std::string command;
+    while (argc > 0) {
+        command += *argv;
         skip(1);
-        if(argc == 0) break;
-        strcat(command," ");
+        if (argc != 0) command += " ";
     }
 
-    fb_queue_command(command,"");
+    fb_queue_command(command.c_str(), "");
     return 0;
 }