nuplayer2: Fix two use-after-frees

- std::ostringstream::c_str returns a string by value, so we can't
  immediately call c_str() on it and assign that to a variable.
- DrmUUID::arrayToHex returns a String8 by value. Similar "can't just
  call string() and assign that to a variable".

Both caught by our static analyzer.

Bug: None
Test: Ran the analyzer again

Change-Id: Ia9e0b2b00ee5150225dd487f3611a3893946fa4b
diff --git a/media/libmediaplayer2/nuplayer2/NuPlayer2Drm.cpp b/media/libmediaplayer2/nuplayer2/NuPlayer2Drm.cpp
index 4853ae1..0e096b0 100644
--- a/media/libmediaplayer2/nuplayer2/NuPlayer2Drm.cpp
+++ b/media/libmediaplayer2/nuplayer2/NuPlayer2Drm.cpp
@@ -148,9 +148,10 @@
     }
 
     uint32_t psshSize = pssh.tellp();
-    const uint8_t* psshPtr = reinterpret_cast<const uint8_t*>(pssh.str().c_str());
-    const char *psshHex = DrmUUID::arrayToHex(psshPtr, psshSize).string();
-    ALOGV("retrieveDrmInfo: MEDIA_DRM_INFO  PSSH: size: %u %s", psshSize, psshHex);
+    std::string psshBase = pssh.str();
+    const auto* psshPtr = reinterpret_cast<const uint8_t*>(psshBase.c_str());
+    ALOGV("retrieveDrmInfo: MEDIA_DRM_INFO  PSSH: size: %u %s", psshSize,
+            DrmUUID::arrayToHex(psshPtr, psshSize).string());
 
     // 1) Write PSSH bytes
     drmInfo.write(reinterpret_cast<const char *>(&psshSize), sizeof(psshSize));