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>

common/hash.c

@@ -214,7 +214,7 @@ static void store_result(struct hash_algo *algo, const u8 *sum,
 			sprintf(str_ptr, "%02x", sum[i]);
 			str_ptr += 2;
 		}
-		str_ptr = '\0';
+		*str_ptr = '\0';
 		setenv(dest, str_output);
 	} else {
 		ulong addr;