Missing NULL Check

Add a safety NULL check in case of MtpDataPacket::putString() with integer arguments, gets called with nullptr as argument.

Bug: 313979409
Change-Id: I3ce755dbc87d74aa7d7d392a0d71129096be3f8b
Test: presubmit
diff --git a/media/mtp/MtpDataPacket.cpp b/media/mtp/MtpDataPacket.cpp
index 42e9078..a2b1c5e 100644
--- a/media/mtp/MtpDataPacket.cpp
+++ b/media/mtp/MtpDataPacket.cpp
@@ -427,6 +427,9 @@
 }
 
 void MtpDataPacket::putString(const uint16_t* string) {
+    if (string == NULL) {
+        return;
+    }
     int count = 0;
     for (int i = 0; i <= MTP_STRING_MAX_CHARACTER_NUMBER; i++) {
         if (string[i])