Stop suppressing pointer-overflow UBSAN errors

* Remove all pointer-overflow suppressions from our UBSAN builds/tests.
* Add `ZSTD_ALLOW_POINTER_OVERFLOW_ATTR` macro to suppress
  pointer-overflow at a per-function level. This is a superior approach
  because it also applies to users who build zstd with UBSAN.
* Add `ZSTD_wrappedPtr{Diff,Add,Sub}()` that use these suppressions.
  The end goal is to only tag these functions with
  `ZSTD_ALLOW_POINTER_OVERFLOW`. But we can start by annoting functions
  that rely on pointer overflow, and gradually transition to using
  these.
* Add `ZSTD_maybeNullPtrAdd()` to simplify pointer addition when the
  pointer may be `NULL`.
* Fix all the fuzzer issues that came up. I'm sure there will be a lot
  more, but these are the ones that came up within a few minutes of
  running the fuzzers, and while running GitHub CI.

tests/fuzz/sequence_compression_api.c

@@ -116,7 +116,7 @@ static size_t decodeSequences(void* dst, size_t nbSequences,
                 }
             }
             for (; j < matchLength; ++j) {
-                op[j] = op[j - generatedSequences[i].offset];
+                op[j] = op[(ptrdiff_t)(j - generatedSequences[i].offset)];
             }
             op += j;
             FUZZ_ASSERT(generatedSequences[i].matchLength == j + k);