fix oob read in getSpelling when node has no ext tokens (cppcheck-opensource#8907)

Author: Nussu06

lib/clangimport.cpp

@@ -422,15 +422,13 @@ std::string clangimport::AstNode::getSpelling() const
     if (nodeType == FunctionDecl || nodeType == CXXConstructorDecl || nodeType == CXXMethodDecl) {
         while (typeIndex >= 0 && mExtTokens[typeIndex][0] != '\'')
             typeIndex--;
-        if (typeIndex <= 0)
-            return "";
     }
     if (nodeType == DeclRefExpr) {
         while (typeIndex > 0 && std::isalpha(mExtTokens[typeIndex][0]))
             typeIndex--;
-        if (typeIndex <= 0)
-            return "";
     }
+    if (typeIndex <= 0)
+        return "";
     const std::string &str = mExtTokens[typeIndex - 1];
     if (startsWith(str,"col:"))
         return "";

test/testclangimport.cpp

@@ -139,6 +139,7 @@ class TestClangImport : public TestFixture {
         TEST_CASE(valueType2);
 
         TEST_CASE(crash);
+        TEST_CASE(crash2);
     }
 
     std::string parse(const char clang[]) {
@@ -1372,6 +1373,13 @@ class TestClangImport : public TestFixture {
                             "  `-CompoundStmt 0x5603791b5700 <col:54, col:55>\n";
         (void)parse(clang); // don't crash
     }
+
+    void crash2() {
+        // getSpelling() indexed mExtTokens[typeIndex - 1] without a lower-bound
+        // check, so a node whose line carries no ext tokens (typeIndex <= 0) read
+        // out of bounds.
+        (void)parse("`-RecordDecl "); // don't crash
+    }
 };
 
 REGISTER_TEST(TestClangImport)