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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.

--- /dev/null

+++ b/bugs-fixed/pfile-overflow.awk

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

+\

\ No newline at end of file

--- /dev/null

+++ b/bugs-fixed/pfile-overflow.ok

@@ -1,0 +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

--- a/main.c

+++ b/main.c

@@ -256,7 +256,7 @@

 char *cursource(void)	/* current source file name */

 {

 	if (npfile > 0)

-		return pfile[curpfile];

+		return pfile[curpfile < npfile ? curpfile : curpfile - 1];

 	else

 		return NULL;

 }

-- 

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