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.

lib/legacy/zstd_v04.c

@@ -16,6 +16,7 @@
 #include <string.h>    /* memcpy */
 
 #include "zstd_v04.h"
+#include "../common/compiler.h"
 #include "../common/error_private.h"
 
 
@@ -3209,6 +3210,7 @@ static size_t ZSTD_decompressContinue(ZSTD_DCtx* ctx, void* dst, size_t maxDstSi
             }
             ctx->stage = ZSTDds_decodeBlockHeader;
             ctx->expected = ZSTD_blockHeaderSize;
+            if (ZSTD_isError(rSize)) return rSize;
             ctx->previousDstEnd = (char*)dst + rSize;
             return rSize;
         }
@@ -3536,8 +3538,8 @@ static size_t ZBUFF_decompressContinue(ZBUFF_DCtx* zbc, void* dst, size_t* maxDs
 unsigned ZBUFFv04_isError(size_t errorCode) { return ERR_isError(errorCode); }
 const char* ZBUFFv04_getErrorName(size_t errorCode) { return ERR_getErrorName(errorCode); }
 
-size_t ZBUFFv04_recommendedDInSize()  { return BLOCKSIZE + 3; }
-size_t ZBUFFv04_recommendedDOutSize() { return BLOCKSIZE; }
+size_t ZBUFFv04_recommendedDInSize(void)  { return BLOCKSIZE + 3; }
+size_t ZBUFFv04_recommendedDOutSize(void) { return BLOCKSIZE; }