Fix possible crash when removing constraints from table definition

This fixes a possible crash when trying to remove a table constraint
from a table definition. The issue was introduced in commit
4e1d1ff49f.

See issue #2670.

src/sql/sqlitetypes.cpp

@@ -505,7 +505,7 @@ void Table::removeConstraints(const StringVector& vStrFields, Constraint::Constr
     for(auto it = m_constraints.begin();it!=m_constraints.end();)
     {
         if((*it)->columnList() == vStrFields && (*it)->type() == type)
-            m_constraints.erase(it++);
+            it = m_constraints.erase(it);
         else
             ++it;
     }