Introduce ZSTD_initCStream_internal()

This is now the regroup point for ZSTD_initCStream*() functions

ZSTD_initCStream_advanced() now properly checks for parameters validity.

Also : added <assert.h> usage inside zstd_compress.c
Needs ZSTD_DEBUG=1 macro to be triggered.
Will be triggered by default from `tests` directory

tests/Makefile

@@ -26,7 +26,9 @@ PYTHON ?= python3
 TESTARTEFACT := versionsTest namespaceTest
 
 
-CPPFLAGS+= -I$(ZSTDDIR) -I$(ZSTDDIR)/common -I$(ZSTDDIR)/compress -I$(ZSTDDIR)/dictBuilder -I$(ZSTDDIR)/deprecated -I$(PRGDIR)
+CPPFLAGS+= -I$(ZSTDDIR) -I$(ZSTDDIR)/common -I$(ZSTDDIR)/compress \
+           -I$(ZSTDDIR)/dictBuilder -I$(ZSTDDIR)/deprecated -I$(PRGDIR) \
+           -DZSTD_DEBUG=1
 CFLAGS  ?= -O3
 CFLAGS  += -g -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow \
            -Wstrict-aliasing=1 -Wswitch-enum -Wdeclaration-after-statement \