MDEV-29578 Fix CONNECT build warnings

Fix a couple of build warnings that fire with CONNECT engine.

storage/connect/bsonudf.cpp

@@ -3571,14 +3571,14 @@ char *bson_item_merge(UDF_INIT *initid, UDF_ARGS *args, char *result,
 	if (!CheckMemory(g, initid, args, 2, false, false, true)) {
 		JTYP  type;
 		BJNX  bnx(g);
-		PBVAL jvp, top = NULL;
+		PBVAL jvp = NULL, top = NULL;
 		PBVAL jsp[2] = {NULL, NULL};
 
 		for (int i = 0; i < 2; i++) {
 			jvp = bnx.MakeValue(args, i, true);
 
 			if (i) {
-				if (jvp->Type != type) {
+				if (jvp && (jvp->Type != type)) {
 					PUSH_WARNING("Argument types mismatch");
 					goto fin;
 				}	// endif type
@@ -5722,14 +5722,14 @@ char *bbin_item_merge(UDF_INIT *initid, UDF_ARGS *args, char *result,
 	if (!CheckMemory(g, initid, args, 2, false, false, true)) {
 		JTYP  type;
 		BJNX  bnx(g);
-		PBVAL jvp, top = NULL;
+		PBVAL jvp = NULL, top = NULL;
 		PBVAL jsp[2] = {NULL, NULL};
 
 		for (int i = 0; i < 2; i++) {
 			if (i) {
 				jvp = bnx.MakeValue(args, i, true);
 
-				if (jvp->Type != type) {
+				if (jvp && (jvp->Type != type)) {
 					PUSH_WARNING("Argument types mismatch");
 					goto fin;
 				}	// endif type

storage/connect/filamdbf.cpp

@@ -244,7 +244,7 @@ PQRYRES DBFColumns(PGLOBAL g, PCSZ dp, PCSZ fn, PTOS topt, bool info)
   int        rc, type, len, field, fields;
   bool       bad, mul;
 	PCSZ       target, pwd;
-  DBFHEADER  mainhead, *hp;
+  DBFHEADER  mainhead, *hp = NULL;
 	DESCRIPTOR thisfield, *tfp;
 	FILE      *infile = NULL;
 	UNZIPUTL  *zutp = NULL;