Fixed parse of cmd string

Before this fix, using cmd() with an erroneous string was triggering
no error. Now this is fixed.

Notice that this is an old bug (introduced in 0a46f15d).
It should be harmless when using MD codes with proper interfaces.

src/core/PlumedMain.cpp

@@ -171,7 +171,10 @@ void PlumedMain::cmd(const std::string & word,void*val){
   std::vector<std::string> words=Tools::getWords(word);
   unsigned nw=words.size();
   if(nw==1) {
-    switch(word_map[word]) {
+    int iword=-1;
+    std::map<std::string, int>::const_iterator it=word_map.find(word);
+    if(it!=word_map.end()) iword=it->second;
+    switch(iword) {
       double d;
       case SETBOX:
         CHECK_INIT(initialized,word);