Fix static analysis warnings of memory leaks.

* False positive static analyzer Warnings:
    AString.cpp:134:11: Potential memory leak [clang-analyzer-unix.Malloc]
    Potential leak of memory pointed to by '___res.mData' in calls to CHECK_LT(start, mSize)
* Simplify AString::clear to work with clang static analyzer.
* Simplify CHECK_OP macros to reduce copying of ___res.

Test: make with WITH_TIDY=1
Change-Id: I7fde7d4d932b37609893eb6f5c529d6727b0da7e