posix: net: Fix undefined behavior

The signedness of the variable caused undefined behavior because the
sign bit is modified when it gets left-shifted.

This fixes that by changing it to an unsigned variable.

Signed-off-by: Daniel Hajjar <daniel.hajjar16@gmail.com>

Author: DanTGL

lib/posix/options/net.c

@@ -19,7 +19,7 @@
 
 in_addr_t inet_addr(const char *cp)
 {
-	int val = 0;
+	unsigned int val = 0;
 	int len = 0;
 	int dots = 0;
 	int digits = 0;