cppcheck: (warning) Possible null pointer dereference: nvh

Move the 'nvh = (void *)buf' assignment after the 'buf == NULL'
check to resolve the warning.  Interestingly, cppcheck 1.88
correctly determines that the existing code is safe, while
cppcheck 1.86 reports the warning.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#9732

module/nvpair/nvpair.c

@@ -2558,7 +2558,7 @@ nvlist_common(nvlist_t *nvl, char *buf, size_t *buflen, int encoding,
 #else
 	int host_endian = 0;
 #endif	/* _LITTLE_ENDIAN */
-	nvs_header_t *nvh = (void *)buf;
+	nvs_header_t *nvh;
 
 	if (buflen == NULL || nvl == NULL ||
 	    (nvs.nvs_priv = (nvpriv_t *)(uintptr_t)nvl->nvl_priv) == NULL)
@@ -2577,6 +2577,7 @@ nvlist_common(nvlist_t *nvl, char *buf, size_t *buflen, int encoding,
 		if (buf == NULL || *buflen < sizeof (nvs_header_t))
 			return (EINVAL);
 
+		nvh = (void *)buf;
 		nvh->nvh_encoding = encoding;
 		nvh->nvh_endian = nvl_endian = host_endian;
 		nvh->nvh_reserved1 = 0;
@@ -2588,6 +2589,7 @@ nvlist_common(nvlist_t *nvl, char *buf, size_t *buflen, int encoding,
 			return (EINVAL);
 
 		/* get method of encoding from first byte */
+		nvh = (void *)buf;
 		encoding = nvh->nvh_encoding;
 		nvl_endian = nvh->nvh_endian;
 		break;