kconfig: sym_expand_string_value: allow for string termination when reallocing

When expanding a parameterised string we may run out of space, this
triggers a realloc.  When computing the new allocation size we do not
allow for the terminating '\0'.  Allow for this when calculating the new
length.
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/kconfig/symbol.c

@@ -875,7 +875,7 @@ const char *sym_expand_string_value(const char *in)
 			symval = sym_get_string_value(sym);
 		}
 
-		newlen = strlen(res) + strlen(symval) + strlen(src);
+		newlen = strlen(res) + strlen(symval) + strlen(src) + 1;
 		if (newlen > reslen) {
 			reslen = newlen;
 			res = realloc(res, reslen);