[NFC][clang][analyzer] Initialize pointer field in StreamOperationEvaluator (#89837)

Add an initializer for StreamSym, which is a pointer. The pointers in
this class are set in the Init function, but all should be initialized
in the constructor to avoid confusion and static verifier hits.

GitOrigin-RevId: cd3e71fb7a5fed9a7e5ee54a2f956ce33b90b90c
1 file changed