[trace] Remove default definitions of weak symbols

Instead of providing a default no-op implementation, check the symbols
for `NULL` before accessing them. Providing a default implementation
doesn't reliably work with dynamic linking. Depending on link order the
default implementations may not be overridden. By skipping the default
implementation, all link order issues are resolved. If the symbols
aren't provided the weak function will be `NULL`.

lib/decompress/zstd_decompress.c

@@ -788,7 +788,7 @@ static size_t ZSTD_setRleBlock(void* dst, size_t dstCapacity,
 static void ZSTD_DCtx_trace_end(ZSTD_DCtx const* dctx, U64 uncompressedSize, U64 compressedSize, unsigned streaming)
 {
 #if ZSTD_TRACE
-    if (dctx->traceCtx) {
+    if (dctx->traceCtx && ZSTD_trace_decompress_end != NULL) {
         ZSTD_Trace trace;
         ZSTD_memset(&trace, 0, sizeof(trace));
         trace.version = ZSTD_VERSION_NUMBER;
@@ -1383,7 +1383,7 @@ size_t ZSTD_decompressBegin(ZSTD_DCtx* dctx)
 {
     assert(dctx != NULL);
 #if ZSTD_TRACE
-    dctx->traceCtx = ZSTD_trace_decompress_begin(dctx);
+    dctx->traceCtx = (ZSTD_trace_decompress_begin != NULL) ? ZSTD_trace_decompress_begin(dctx) : 0;
 #endif
     dctx->expected = ZSTD_startingInputLength(dctx->format);  /* dctx->format must be properly set */
     dctx->stage = ZSTDds_getFrameHeaderSize;