Fix invalid memory access in S3 comms (#3681)

In the ros3 VFD, passing an empty string parameter to an internal
API call could result in accessing the -1th element of a string.
This would cause failures on big-endian systems like s390x.

This parameter is now checked before writing to the string.

Fixes GitHub #1168

src/H5FDs3comms.c

@@ -1713,7 +1713,8 @@ H5FD_s3comms_aws_canonical_request(char *canonical_request_dest, int _cr_size, c
     } /* end while node is not NULL */
 
     /* remove trailing ';' from signed headers sequence */
-    signed_headers_dest[strlen(signed_headers_dest) - 1] = '\0';
+    if (*signed_headers_dest != '\0')
+        signed_headers_dest[strlen(signed_headers_dest) - 1] = '\0';
 
     /* append signed headers and payload hash
      * NOTE: at present, no HTTP body is handled, per the nature of