Grow the lex token buffer in one more place

when parsing key pairs, if the '=' character fell at max_token
a protective INSIST preventing buffer overrun could be triggered.
Attempt to grow the buffer immediately before the INSIST.

Also removed an unnecessary INSIST on the opening double quote
of key buffer pair.

(cherry picked from commit 4c356d2770)

lib/isc/lex.c

@@ -670,6 +670,13 @@ isc_lex_gettoken(isc_lex_t *lex, unsigned int options, isc_token_t *tokenp) {
 		case lexstate_string:
 			if (!escaped && c == '=' &&
 			    (options & ISC_LEXOPT_VPAIR) != 0) {
+				if (remaining == 0U) {
+					result = grow_data(lex, &remaining,
+							   &curr, &prev);
+					if (result != ISC_R_SUCCESS) {
+						goto done;
+					}
+				}
 				INSIST(remaining > 0U);
 				*curr++ = c;
 				*curr = '\0';
@@ -682,7 +689,6 @@ isc_lex_gettoken(isc_lex_t *lex, unsigned int options, isc_token_t *tokenp) {
 			if (state == lexstate_vpairstart) {
 				if (c == '"' &&
 				    (options & ISC_LEXOPT_QVPAIR) != 0) {
-					INSIST(remaining > 0U);
 					no_comments = true;
 					state = lexstate_qvpair;
 					break;