build: Fix default values for some SANITIZER options

2 of the sanitizers didn't have default values so in systems that
don't support sanitizers menuselect would spit out warnings.  They
were harmless but confusing.  They've now been set to "0".

Change-Id: I08dc495e3b83f1feac3160b421f538c375fc5d58

configure

@@ -18585,7 +18585,7 @@ if ac_fn_c_try_compile "$LINENO"; then :
 $as_echo "yes" >&6; }
 	AST_ADDRESS_SANITIZER=1
 else
-  AST_ADDRESS_SANITIZER=
+  AST_ADDRESS_SANITIZER=0
 	{ $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }
 
@@ -18617,7 +18617,7 @@ if ac_fn_c_try_compile "$LINENO"; then :
 $as_echo "yes" >&6; }
 	AST_THREAD_SANITIZER=1
 else
-  AST_THREAD_SANITIZER=
+  AST_THREAD_SANITIZER=0
 	{ $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }
 

configure.ac

@@ -1113,7 +1113,7 @@ AC_COMPILE_IFELSE(
 	[AC_LANG_PROGRAM([], [int x = 1;])],
 	AC_MSG_RESULT(yes)
 	[AST_ADDRESS_SANITIZER=1],
-	[AST_ADDRESS_SANITIZER=]
+	[AST_ADDRESS_SANITIZER=0]
 	AC_MSG_RESULT(no)
 )
 CFLAGS="${saved_sanitize_CFLAGS}"
@@ -1129,7 +1129,7 @@ AC_COMPILE_IFELSE(
 	[AC_LANG_PROGRAM([], [int x = 1;])],
 	AC_MSG_RESULT(yes)
 	[AST_THREAD_SANITIZER=1],
-	[AST_THREAD_SANITIZER=]
+	[AST_THREAD_SANITIZER=0]
 	AC_MSG_RESULT(no)
 )
 CFLAGS="${saved_sanitize_CFLAGS}"