Avoid accessing pfile[] out of bounds on syntax error at EOF. (#90)

When awk reaches EOF parsing the program file, curpfile is incremented.
However, cursource() uses curpfile without checking it against npfile
which can cause an out of bounds access of pfile[] if there is a syntax
error at the end of the program file.

bugs-fixed/pfile-overflow.awk

@@ -0,0 +1 @@
+\

bugs-fixed/pfile-overflow.ok

@@ -0,0 +1,4 @@
+../a.out: syntax error at source line 1 source file pfile-overflow.awk
+ context is
+	 >>>  <<< 
+../a.out: bailing out at source line 1 source file pfile-overflow.awk

main.c

@@ -256,7 +256,7 @@ int pgetc(void)		/* get 1 character from awk program */
 char *cursource(void)	/* current source file name */
 {
 	if (npfile > 0)
-		return pfile[curpfile];
+		return pfile[curpfile < npfile ? curpfile : curpfile - 1];
 	else
 		return NULL;
 }