Fix an overflow case in fdt_offset_ptr() detected by GCC 4.3.

Using Gcc 4.3 detected this problem:

    ../dtc/libfdt/fdt.c: In function 'fdt_next_tag':
    ../dtc/libfdt/fdt.c:82: error: assuming signed overflow does not
    occur when assuming that (X + c) < X is always false

To fix the problem, treat the offset as an unsigned int.

The problem report and proposed fix were provided
by Steve Papacharalambous <stevep@freescale.com>.
Signed-off-by: NJon Loeliger <jdl@freescale.com>

include/libfdt.h

@@ -122,7 +122,7 @@
 /* Low-level functions (you probably don't need these)                */
 /**********************************************************************/
 
-const void *fdt_offset_ptr(const void *fdt, int offset, int checklen);
+const void *fdt_offset_ptr(const void *fdt, int offset, unsigned int checklen);
 static inline void *fdt_offset_ptr_w(void *fdt, int offset, int checklen)
 {
 	return (void *)(uintptr_t)fdt_offset_ptr(fdt, offset, checklen);

libfdt/fdt.c

@@ -78,7 +78,7 @@ int fdt_check_header(const void *fdt)
 	return 0;
 }
 
-const void *fdt_offset_ptr(const void *fdt, int offset, int len)
+const void *fdt_offset_ptr(const void *fdt, int offset, unsigned int len)
 {
 	const char *p;