[NFC] [clang-tidy] Fix potential SA issues. (llvm#170289)

This patch addresses issues identified by the static analyzers, which
appear to be legitimate problems.

`FloatLoopCounterCheck.cpp`: "Dereferencing a pointer that might be
`nullptr` FS when calling `getInc`".
`ProBoundsAvoidUncheckedContainerAccessCheck.cpp`: "Dereferencing a
pointer that might be `nullptr Callee` when calling `getBeginLoc`".
`ExpandModularHeadersPPCallbacks.cpp`: Non-static class member
`CurrentToken.Flags` is not initialized in this constructor nor in any
functions that it calls. (line llvm#101).

Author: zahiraam

clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.h

@@ -137,7 +137,7 @@ class ExpandModularHeadersPPCallbacks : public PPCallbacks {
   std::unique_ptr<Preprocessor> PP;
   bool EnteredMainFile = false;
   bool StartedLexing = false;
-  Token CurrentToken;
+  Token CurrentToken = Token();
 };
 
 } // namespace tooling

clang-tools-extra/clang-tidy/bugprone/FloatLoopCounterCheck.cpp

@@ -31,6 +31,7 @@ void FloatLoopCounterCheck::registerMatchers(MatchFinder *Finder) {
 
 void FloatLoopCounterCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *FS = Result.Nodes.getNodeAs<ForStmt>("for");
+  assert(FS && "FS should not be null");
 
   diag(FS->getInc()->getBeginLoc(), "loop induction expression should not have "
                                     "floating-point type")

clang-tools-extra/clang-tidy/cppcoreguidelines/ProBoundsAvoidUncheckedContainerAccessCheck.cpp

@@ -176,7 +176,7 @@ void ProBoundsAvoidUncheckedContainerAccessCheck::check(
     }
   } else if (const auto *MCE = dyn_cast<CXXMemberCallExpr>(MatchedExpr)) {
     // Case: a.operator[](i) or a->operator[](i)
-    const auto *Callee = dyn_cast<MemberExpr>(MCE->getCallee());
+    const auto *Callee = cast<MemberExpr>(MCE->getCallee());
 
     if (FixMode == At) {
       // Cases: a.operator[](i) => a.at(i) and a->operator[](i) => a->at(i)