fix minor compilation condition

harness.c is not designed to pass  -Wdeclaration-after-statement

doc/educational_decoder/Makefile

@@ -6,10 +6,10 @@ CPPFLAGS += -I$(ZSTDDIR) -I$(ZSTDDIR)/common -I$(ZSTDDIR)/compress \
             -I$(ZSTDDIR)/dictBuilder -I$(ZSTDDIR)/deprecated -I$(PRGDIR)
 CFLAGS   ?= -O3
 CFLAGS   += -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow                 \
-            -Wstrict-aliasing=1 -Wswitch-enum -Wdeclaration-after-statement \
-            -Wstrict-prototypes -Wundef                                     \
+            -Wstrict-aliasing=1 -Wswitch-enum                               \
+            -Wredundant-decls -Wstrict-prototypes -Wundef                   \
             -Wvla -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings      \
-            -Wredundant-decls
+            -std=c99
 CFLAGS   += $(DEBUGFLAGS)
 CFLAGS   += $(MOREFLAGS)
 FLAGS     = $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) $(MULTITHREAD_LDFLAGS)

doc/educational_decoder/harness.c

@@ -33,7 +33,7 @@ size_t read_file(const char *path, u8 **ptr) {
     }
 
     fseek(f, 0L, SEEK_END);
-    size_t size = ftell(f);
+    size_t size = (size_t)ftell(f);
     rewind(f);
 
     *ptr = malloc(size);

doc/educational_decoder/zstd_decompress.c

@@ -833,6 +833,7 @@ static size_t decode_literals_compressed(frame_context_t *const ctx,
         // bits (0-1023)."
         num_streams = 1;
     // Fall through as it has the same size format
+        /* fallthrough */
     case 1:
         // "4 streams. Both Compressed_Size and Regenerated_Size use 10 bits
         // (0-1023)."