libfdt: Fix bugs with unchecked usage of fdt_num_mem_rsv()

fdt_num_mem_rsv() can return an error if the memory reservation block
is not properly terminated with a (0, 0) entry.  However several other
places in libfdt called it without checking for error returns, and could
therefore return strange results, or in the case of fdt_open_into()
crash.

Fix this by always checking the return value.  Add some addition tests to
catch this bug.

Reported-by: Moshe Strauss <moshestrauss10@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/truncated_memrsv.c

@@ -15,9 +15,12 @@
 #include "tests.h"
 #include "testdata.h"
 
+#define SPACE		65536
+
 int main(int argc, char *argv[])
 {
 	void *fdt = &truncated_memrsv;
+	void *buf;
 	int err;
 	uint64_t addr, size;
 
@@ -46,5 +49,11 @@ int main(int argc, char *argv[])
 		FAIL("fdt_get_mem_rsv(1) returned %d instead of -FDT_ERR_BADOFFSET",
 		     err);
 
+	buf = xmalloc(SPACE);
+	err = fdt_open_into(fdt, buf, SPACE);
+	if (err != -FDT_ERR_TRUNCATED)
+		FAIL("fdt_open_into() returned %d instead of -FDT_ERR_TRUNCATED",
+		     err);
+
 	PASS();
 }