Fix null pointer segfault when calling dump() on a DeclStmt containing a VarDecl.

[jordan_rose]

Question: why do we not have a SourceManager here?

[alexmc]

If you call Stmt::dump(), it creates an ASTDumper without a SourceManager: ASTDumper.cpp:2163 http://clang.llvm.org/doxygen/ASTDumper_8cpp_source.html#l02162

Note that the other place SM is dereferenced in that file (ASTDumper.cpp:189) we check SM for null before using it.

[jordan_rose]

Okay, thanks for the explanation. Please commit.

[alexmc]

Submitted as r207867

lib/AST/ASTDumper.cpp

 }
 
 void ASTDumper::dumpPointer(const void *Ptr) {
   ColorScope Color(*this, AddressColor);
   OS << ' ' << Ptr;
 }
 
 void ASTDumper::dumpLocation(SourceLocation Loc) {
+  if (!SM)
+    return;
+
   ColorScope Color(*this, LocationColor);
   SourceLocation SpellingLoc = SM->getSpellingLoc(Loc);
 
   // The general format we print out is filename:line:col, but we drop pieces
   // that haven't changed since the last loc printed.
   PresumedLoc PLoc = SM->getPresumedLoc(SpellingLoc);
 
   if (PLoc.isInvalid()) {