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/fuzzer.c

@@ -328,7 +328,7 @@ static void FUZ_decodeSequences(BYTE* dst, ZSTD_Sequence* seqs, size_t seqsSize,
 
         if (seqs[i].offset != 0) {
             for (j = 0; j < seqs[i].matchLength; ++j)
-                dst[j] = dst[j - seqs[i].offset];
+                dst[j] = dst[(ptrdiff_t)(j - seqs[i].offset)];
             dst += seqs[i].matchLength;
             src += seqs[i].matchLength;
             size -= seqs[i].matchLength;
@@ -3684,11 +3684,13 @@ static int basicUnitTests(U32 const seed, double compressibility)
 
         /* Test with block delimiters roundtrip */
         seqsSize = ZSTD_generateSequences(cctx, seqs, srcSize, src, srcSize);
+        CHECK_Z(seqsSize);
         FUZ_decodeSequences(decoded, seqs, seqsSize, src, srcSize, ZSTD_sf_explicitBlockDelimiters);
         assert(!memcmp(CNBuffer, compressedBuffer, srcSize));
 
         /* Test no block delimiters roundtrip */
         seqsSize = ZSTD_mergeBlockDelimiters(seqs, seqsSize);
+        CHECK_Z(seqsSize);
         FUZ_decodeSequences(decoded, seqs, seqsSize, src, srcSize, ZSTD_sf_noBlockDelimiters);
         assert(!memcmp(CNBuffer, compressedBuffer, srcSize));