Handle null results in execScalar()

It now takes an `nNullSentinel` parameter similar to `getIntField()` and returns
it when the result of the scalar query is null.

Closes #16.

Author: mqudsi

src/CppSQLite3.cpp

@@ -1350,7 +1350,7 @@ CppSQLite3Query CppSQLite3DB::execQuery(const char* szSQL)
 }
 
 
-int CppSQLite3DB::execScalar(const char* szSQL)
+int CppSQLite3DB::execScalar(const char* szSQL, int nNullSentinel)
 {
     CppSQLite3Query q = execQuery(szSQL);
 
@@ -1361,7 +1361,7 @@ int CppSQLite3DB::execScalar(const char* szSQL)
                                 DONT_DELETE_MSG);
     }
 
-    return atoi(q.fieldValue(0));
+    return q.getIntField(0, nNullSentinel);
 }
 
 

src/CppSQLite3.h

@@ -309,7 +309,7 @@ class CppSQLite3DB
 
     CppSQLite3Query execQuery(const char* szSQL);
 
-    int execScalar(const char* szSQL);
+    int execScalar(const char* szSQL, int nNullSentinel = 0);
 
     CppSQLite3Table getTable(const char* szSQL);