helper/command: fix clang static analyzer warning

helper/command: fix clang static analyzer warning

Warning:	line 955, column 3
Argument to free() is the address of a global variable, which
is not memory allocated by malloc()

It is definitely a false alarm. Simplify concatenation of arguments
and allocate a string always to silence the warning.

Change-Id: I5ac4cc610fc35224df0b16ef4f7102700363249f
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5904
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>