Fix a UB when fdt_get_string return null

When fdt_get_string return null, `namep` is not correctly reset.
From the document of `fdt_getprop_by_offset`, the parameter `namep` will
be always overwritten (that is, it will be overwritten without exception
of error occurance).

As for the caller (like
https://github.com/topjohnwu/Magisk/blob/e097c097feb881f6097b6d1dc346f310bc92f5d6/native/jni/magiskboot/dtb.cpp#L42),
the code may be like:
```cpp
size_t size;
const char *name;
auto *value = fdt_getprop_by_offset(fdt, prop, &name, &size);
```
and if `value == nullptr`, `size` is also be overwritten correctly but
`name` is not, which is quite inconsistent.

This commit makes sure `name` and `size` behavior consistently (reset to
reasonable value) when error occurs.

Signed-off-by: LoveSy <shana@zju.edu.cn>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Author: yujincheng08

libfdt/fdt_ro.c

@@ -481,12 +481,12 @@ const void *fdt_getprop_by_offset(const void *fdt, int offset,
 		if (!can_assume(VALID_INPUT)) {
 			name = fdt_get_string(fdt, fdt32_ld_(&prop->nameoff),
 					      &namelen);
+			*namep = name;
 			if (!name) {
 				if (lenp)
 					*lenp = namelen;
 				return NULL;
 			}
-			*namep = name;
 		} else {
 			*namep = fdt_string(fdt, fdt32_ld_(&prop->nameoff));
 		}