fix px4_getopt: correctly handle options that take an argument, but no argument is given

This lead to a segfault, for example 'logger start -r'

src/platforms/common/px4_getopt.c

@@ -165,6 +165,9 @@ __EXPORT int px4_getopt(int argc, char *argv[], const char *options, int *myopti
 
 		if (takesarg) {
 			*myoptarg = argv[*myoptind];
+			if (!*myoptarg) { //Error: option takes an argument, but there is no more argument
+				return -1;
+			}
 			*myoptind += 1;
 		}