Fix #14876 Nullptr dereference in usingMatch() (#8678)

Root cause is here:
https://github.com/cppcheck-opensource/cppcheck/blob/86f4c91741690386cd5917ccd8120624d9473ad5/lib/tokenize.cpp#L3108

Co-authored-by: chrchr-github <noreply@github.com>

Author: chrchr-github

lib/tokenize.cpp

@@ -2720,6 +2720,9 @@ namespace {
             return false;
         }
 
+        if (!tok->tokAt(-1))
+            return false;
+
         // skip other using with this name
         if (tok1->strAt(-1) == "using") {
             // fixme: this is wrong

test/testsimplifyusing.cpp

@@ -77,6 +77,7 @@ class TestSimplifyUsing : public TestFixture {
         TEST_CASE(simplifyUsing37);
         TEST_CASE(simplifyUsing38);
         TEST_CASE(simplifyUsing39);
+        TEST_CASE(simplifyUsing40);
 
         TEST_CASE(simplifyUsing8970);
         TEST_CASE(simplifyUsing8971);
@@ -940,6 +941,13 @@ class TestSimplifyUsing : public TestFixture {
         ASSERT_EQUALS("", errout_str());
     }
 
+    void simplifyUsing40() {
+        const char code[] = "uint8_t f();\n" // #14876
+                            "using ::std::uint8_t;";
+        const char expected[] = "uint8_t f ( ) ;";
+        ASSERT_EQUALS(expected, tok(code));
+    }
+
     void simplifyUsing8970() {
         const char code[] = "using V = std::vector<int>;\n"
                             "struct A {\n"