awk: initialize records fully in recinit()

when using records in BEGIN, we would read
from the field table before we read into it;
this ensures that the fields are an empty
string before we start touching their contents.

sys/src/cmd/awk/lib.c

@@ -54,11 +54,13 @@ static Cell dollar1 = { OCELL, CFLD, nil, "", 0.0, FLD|STR|DONTFREE };
 
 void recinit(unsigned int n)
 {
+	assert(n > 0);
 	record = (char *) malloc(n);
 	fields = (char *) malloc(n);
 	fldtab = (Cell **) malloc((nfields+1) * sizeof(Cell *));
 	if (record == nil || fields == nil || fldtab == nil)
 		FATAL("out of space for $0 and fields");
+	record[0] = '\0';
 	fldtab[0] = (Cell *) malloc(sizeof (Cell));
 	*fldtab[0] = dollar0;
 	fldtab[0]->sval = record;