ensure callback is only called once

lib/command.js

@@ -188,7 +188,17 @@ module.exports = function (proto) {
         err = new Error("gm().stream() or gm().write() with a non-readable " +
                         "stream. Pass \"{bufferStream: true}\" to identify() " +
                         "or getter (size, format, etc...)");
-        return callback.call(this, err);
+        return cb(this, err);
+      }
+
+      // handle streaming to gm errors when gm/imageMagick is not installed
+      proc.stdin.on('error', handlerr);
+      proc.stdout.on('error', handlerr);
+      function handlerr (err) {
+        if ('EPIPE' == err.code && 'write' == err.syscall) {
+          err.message = missingBinMsg(self) + err.message;
+        }
+        cb(this, err);
       }
 
       self.sourceStream.pipe(proc.stdin);
@@ -229,11 +239,11 @@ module.exports = function (proto) {
           err.code = code;
           err.signal = signal;
         };
-        callback.call(this, err, stdout, stderr, cmd);
+        cb(this, err, stdout, stderr, cmd);
         stdout = stderr = onOut = onErr = onExit = null;
       });
     } else {
-      callback.call(this, null, proc.stdout, proc.stderr, cmd);
+      cb(this, null, proc.stdout, proc.stderr, cmd);
     }
 
     return this;

test/index.js

@@ -29,7 +29,7 @@ function test (imagemagick) {
 
 function finish (filename) {
   return function (err) {
-    if (err) throw new Error(err);
+    if (err) throw err;
 
     --pending;
     process.stdout.write('\033[2K');