xenpmd: make 32 bit gcc 8.1 non-debug build work

32 bit gcc 8.1 non-debug build yields:

xenpmd.c:354:23: error: '%02x' directive output may be truncated writing between 2 and 8 bytes into a region of size 3 [-Werror=format-truncation=]
     snprintf(val, 3, "%02x",
                       ^~~~
xenpmd.c:354:22: note: directive argument in the range [40, 2147483778]
     snprintf(val, 3, "%02x",
                      ^~~~~~
xenpmd.c:354:5: note: 'snprintf' output between 3 and 9 bytes into a destination of size 3
     snprintf(val, 3, "%02x",
     ^~~~~~~~~~~~~~~~~~~~~~~~
              (unsigned int)(9*4 +
              ~~~~~~~~~~~~~~~~~~~~
                             strlen(info->model_number) +
                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                             strlen(info->serial_number) +
                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                             strlen(info->battery_type) +
                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                             strlen(info->oem_info) + 4));
                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

All info->* used in calculation are 32 bytes long, and the parsing
code makes sure they are null-terminated, so the end result of the
expression won't exceed 255, which should be able to be fit into 3
bytes in hexadecimal format.

Add an assertion to make gcc happy.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/xenpmd/xenpmd.c

@@ -40,6 +40,7 @@
 #include <unistd.h>
 #include <sys/stat.h>
 #include <xenstore.h>
+#include <assert.h>
 
 /* #define RUN_STANDALONE */
 #define RUN_IN_SIMULATE_MODE
@@ -345,18 +346,17 @@ void write_ulong_lsb_first(char *temp_val, unsigned long val)
 void write_battery_info_to_xenstore(struct battery_info *info)
 {
     char val[1024], string_info[256];
+    unsigned int len;
 
     xs_mkdir(xs, XBT_NULL, "/pm");
    
     memset(val, 0, 1024);
     memset(string_info, 0, 256);
     /* write 9 dwords (so 9*4) + length of 4 strings + 4 null terminators */
-    snprintf(val, 3, "%02x", 
-             (unsigned int)(9*4 +
-                            strlen(info->model_number) +
-                            strlen(info->serial_number) +
-                            strlen(info->battery_type) +
-                            strlen(info->oem_info) + 4));
+    len = 9 * 4 + strlen(info->model_number) + strlen(info->serial_number) +
+          strlen(info->battery_type) + strlen(info->oem_info) + 4;
+    assert(len < 255);
+    snprintf(val, 3, "%02x", len);
     write_ulong_lsb_first(val+2, info->present);
     write_ulong_lsb_first(val+10, info->design_capacity);
     write_ulong_lsb_first(val+18, info->last_full_capacity);