common: hash: zero end the string instead of the pointer

if algo->digest_size is zero nothing is set in the str_output
buffer. An attempt is made to zero end the buffer, but the
pointer to the buffer is set to zero instead. I am unaware if
it causes any actual problems, but solves the following warning:

common/hash.c:217:13: warning: expression which evaluates to zero treated as
a null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
                str_ptr = '\0';
                          ^~~~

cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>