[cmdline] Fix image command-line construction for zero-length argument lists

This fixes a bug introduced in commit 4c85017.

src/hci/commands/image_cmd.c

 	/* Determine total length of command line */
 	len = 1; /* NUL */
 	for ( i = 0 ; i < nargs ; i++ )
-		len += ( 1 /* space */ + strlen ( args[i] ) );
+		len += ( 1 /* possible space */ + strlen ( args[i] ) );
 
 	{
 		char buf[len];
 		char *ptr = buf;
 
 		/* Assemble command line */
-		for ( i = 0 ; i < nargs ; i++ )
-			ptr += sprintf ( ptr, " %s", args[i] );
-		assert ( ptr == ( buf + len - 1 ) );
+		buf[0] = '\0';
+		for ( i = 0 ; i < nargs ; i++ ) {
+			ptr += sprintf ( ptr, "%s%s", ( i ? " " : "" ),
+					 args[i] );
+		}
+		assert ( ptr < ( buf + len ) );
 
-		return image_set_cmdline ( image, &buf[1] );
+		return image_set_cmdline ( image, buf );
 	}
 }