[Pal] Enable -Wnull-dereference

Pal/lib/Makefile

@@ -4,6 +4,7 @@ CC	= gcc
 AR	= ar rcs
 
 CFLAGS	= -Wall -fPIC -O2 -std=gnu99 -fgnu89-inline -U_FORTIFY_SOURCE \
+	  $(call cc-option,-Wnull-dereference) \
 	  -fno-omit-frame-pointer \
 	  -fno-stack-protector -fno-builtin
 ARFLAGS	=

Pal/src/host/Linux-SGX/Makefile.am

@@ -10,7 +10,7 @@ CFLAGS	= -Wall -fPIC -O2 -maes -std=c11 -U_FORTIFY_SOURCE \
 	  -fno-omit-frame-pointer \
 	  -fno-stack-protector -fno-builtin -DIN_ENCLAVE
 
-EXTRAFLAGS = -Wextra
+EXTRAFLAGS = -Wextra $(call cc-option,-Wnull-dereference)
 
 CFLAGS += $(EXTRAFLAGS)
 

Pal/src/host/Linux/Makefile.am

@@ -10,7 +10,7 @@ LD	= ld
 CFLAGS	= -Wall -fPIC -O2 -std=c11 -U_FORTIFY_SOURCE \
 	  -fno-stack-protector -fno-builtin
 
-EXTRAFLAGS = -Wextra -Wno-sign-compare
+EXTRAFLAGS = -Wextra -Wno-sign-compare $(call cc-option,-Wnull-dereference)
 
 CFLAGS += $(EXTRAFLAGS)