Fixed #1720 (segmentation fault of cppcheck)

lib/checkother.cpp

@@ -523,6 +523,7 @@ void Variables::alias(unsigned int varid1, unsigned int varid2)
     {
         VariableUsage *temp = find(*i);
 
+        if (temp)
             temp->_aliases.erase(var1->_name->varId());
     }
 

test/testunusedvar.cpp

@@ -1147,8 +1147,7 @@ private:
 
     void localvar17() // ticket #1720
     {
-        // FIXME: Don't crash when checking the code below!
-        /*
+        // Don't crash when checking the code below!
         functionVariableUsage("void foo()\n"
                               "{\n"
                               "    struct DATA *data;\n"
@@ -1160,7 +1159,7 @@ private:
                               "    data->info = k;\n"
                               "    line_start = ptr;\n"
                               "}\n");
-        */
+        ASSERT_EQUALS("[test.cpp:6]: (style) Variable 'line_start' is assigned a value that is never used\n", errout.str());
     }
 
     void localvaralias1()