Print an error message for negative NF values

bugs-fixed/README

@@ -25,3 +25,5 @@ and also if CONVFMT changed.
 Instead, it should convert the value to numeric and give that value.
 
 X. nf-self-assign: "NF = NF" wouldn't force the record to be rebuilt.
+
+X. negative-nf: Setting NF to a negative value caused a segmentation fault.

bugs-fixed/negative-nf.awk

@@ -0,0 +1 @@
+BEGIN { NF = -5; }

bugs-fixed/negative-nf.ok

@@ -0,0 +1,2 @@
+./a.out: cannot set NF to a negative value
+ source line number 1

lib.c

@@ -390,6 +390,8 @@ void newfld(int n)	/* add field n after end of existing lastfld */
 
 void setlastfld(int n)	/* set lastfld cleaning fldtab cells if necessary */
 {
+	if (n < 0)
+		FATAL("cannot set NF to a negative value");
 	if (n > nfields)
 		growfldtab(n);