ptx: fix an invalid heap reference with short --width

* src/ptx.c (fix_output_parameters): Ensure line_width doesn't
go negative, which can happen when the --width is less
than the --gap-size.
* tests/misc/ptx-overrun.sh: Add a test case that triggers
with ASAN.  (Note the longer filename is needed to trigger).
Fixes http://bugs.gnu.org/25011

1 files changed, 2 insertions, 0 deletions

diff --git a/src/ptx.c b/src/ptx.c
index c3b60dfa5..d1896789b 100644
--- a/src/ptx.c
+++ b/src/ptx.c
@@ -1235,6 +1235,8 @@ fix_output_parameters (void)
 
   if ((auto_reference || input_reference) && !right_reference)
     line_width -= reference_max_width + gap_size;
+  if (line_width < 0)
+    line_width = 0;
 
   /* The output lines, minimally, will contain from left to right a left
      context, a gap, and a keyword followed by the right context with no