EventManager: store event handlers in an unordered_map instead of a vector

If an event handler add or removes an event from the manager, it may then
be moved in the vector, and if after that it access any of it's members
(through this), it results in an invalid memory access.

src/event_manager.cc

@@ -15,7 +15,7 @@ void EventManager::watch(int fd, EventHandler handler)
         throw runtime_error("fd already watched");
 
     m_events.push_back(pollfd{ fd, POLLIN | POLLPRI, 0 });
-    m_handlers.push_back(std::move(handler));
+    m_handlers.emplace(fd, std::move(handler));
 }
 
 void EventManager::unwatch(int fd)
@@ -25,7 +25,7 @@ void EventManager::unwatch(int fd)
         if (m_events[i].fd == fd)
         {
             m_events.erase(m_events.begin() + i);
-            m_handlers.erase(m_handlers.begin() + i);
+            m_handlers.erase(fd);
             return;
         }
     }
@@ -39,7 +39,7 @@ void EventManager::handle_next_events()
     {
         if ((res > 0 and m_events[i].revents) or
             contains(m_forced, m_events[i].fd))
-            m_handlers[i](m_events[i].fd);
+            m_handlers[m_events[i].fd](m_events[i].fd);
     }
     m_forced.clear();
 }

src/event_manager.hh

@@ -2,6 +2,7 @@
 #define event_manager_hh_INCLUDED
 
 #include <poll.h>
+#include <unordered_map>
 
 #include "utils.hh"
 
@@ -24,7 +25,7 @@ public:
 
 private:
     std::vector<pollfd> m_events;
-    std::vector<EventHandler> m_handlers;
+    std::unordered_map<int, EventHandler> m_handlers;
     std::vector<int>    m_forced;
 };