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_v02.c

@@ -11,6 +11,7 @@
 
 #include <stddef.h>    /* size_t, ptrdiff_t */
 #include "zstd_v02.h"
+#include "../common/compiler.h"
 #include "../common/error_private.h"
 
 
@@ -71,20 +72,6 @@ extern "C" {
 #include <string.h>    /* memcpy */
 
 
-/******************************************
-*  Compiler-specific
-******************************************/
-#if defined(__GNUC__)
-#  define MEM_STATIC static __attribute__((unused))
-#elif defined (__cplusplus) || (defined (__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) /* C99 */)
-#  define MEM_STATIC static inline
-#elif defined(_MSC_VER)
-#  define MEM_STATIC static __inline
-#else
-#  define MEM_STATIC static  /* this version may generate warnings for unused static functions; disable the relevant warning */
-#endif
-
-
 /****************************************************************
 *  Basic Types
 *****************************************************************/
@@ -3431,6 +3418,7 @@ static size_t ZSTD_decompressContinue(ZSTD_DCtx* ctx, void* dst, size_t maxDstSi
         }
         ctx->phase = 1;
         ctx->expected = ZSTD_blockHeaderSize;
+        if (ZSTD_isError(rSize)) return rSize;
         ctx->previousDstEnd = (void*)( ((char*)dst) + rSize);
         return rSize;
     }