MDEV-30713 field length handling for CONNECT engine

fp->field_length was unsigned and therefore the negative
condition around it.

Backport of cc182aca93 fixes it, however to correct the
consistent use of types pcf->Length needs to be unsigned
too.

At one point pcf->Precision is assigned from pcf->Length so
that's also unsigned.

GetTypeSize is assigned to length and has a length argument.
A -1 default value seemed dangerious to case, so at least 0
should assert if every hit.

storage/connect/tabext.cpp

@@ -466,7 +466,7 @@ bool TDBEXT::MakeSQL(PGLOBAL g, bool cnt)
 
 	if (Quote) {
 		// Tabname can have both database and table identifiers, we need to parse
-		if (res= strstr(buf, "."))
+		if ((res= strstr(buf, ".")))
 		{
 			// Parse schema
 			my_len= res - buf + 1;