nfc_context_new(): replace err() by return, problem still in nfc_init()!!

Not a good idea to call exit() from a library...
Problem is now moved to
void nfc_init() calling exit()
This requires a change in API to return error rather than exiting...

libnfc/nfc-internal.c

@@ -68,7 +68,7 @@ nfc_context_new(void)
   nfc_context *res = malloc(sizeof(*res));
 
   if (!res) {
-    err(EXIT_FAILURE, "nfc_context_new: malloc");
+    return NULL;
   }
 
   // Set default context values

libnfc/nfc.c

@@ -172,7 +172,11 @@ void
 nfc_init(nfc_context **context)
 {
   *context = nfc_context_new();
-
+  if (!context) {
+    perror("malloc");
+    // TODO: not a good idea to call exit() from a library, we should change API and return error
+    exit(EXIT_FAILURE);
+  }
   if (!nfc_drivers)
     nfc_drivers_init();
 }