readCString: no ubsan sub-overflow am: 1086548c6c am: 13788cbc36
am: 1a4c63ee1a

Change-Id: Ib3fa336769dbd8a95ced8c2a98de05af815ea452

Parcel.cpp

diff --git a/Parcel.cpp b/Parcel.cpp
index 6e9bd1e..096913f 100644
--- a/Parcel.cpp
+++ b/Parcel.cpp
@@ -1332,8 +1332,8 @@
 
 const char* Parcel::readCString() const
 {
-    const size_t avail = mDataSize-mDataPos;
-    if (avail > 0) {
+    if (mDataPos < mDataSize) {
+        const size_t avail = mDataSize-mDataPos;
         const char* str = reinterpret_cast<const char*>(mData+mDataPos);
         // is the string's trailing NUL within the parcel's valid bounds?
         const char* eos = reinterpret_cast<const char*>(memchr(str, 0, avail));