[PATCH] Assert that 'length' > 0 on infcmd.c:construct_inferior_arguments

[Sergio Durigan Junior <sergiodj@redhat.com>]

While testing a GCC 10 build of our git HEAD, I noticed an error
triggered by -Werror-stringop on
infcmd.c:construct_inferior_arguments.  One of the things the function
does is calculate the length of the string that will hold the
inferior's arguments.  GCC warns us that 'length' can be 0, which can
lead to undesired behaviour:

../../gdb/infcmd.c: In function 'char* construct_inferior_arguments(int, char**)':
../../gdb/infcmd.c:369:17: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
  369 |       result[0] = '\0';
      |       ~~~~~~~~~~^~~~~~
../../gdb/infcmd.c:368:33: note: at offset 0 to an object with size 0 allocated by 'xmalloc' here
  368 |       result = (char *) xmalloc (length);
      |                         ~~~~~~~~^~~~~~~~

The solution here is to explicit check that 'length' is greater than
0.  Since we're dealing with 'argc', I think it's pretty much
guaranteed that it's going to be at least 1.

Tested by rebuilding.

OK?

gdb/ChangeLog:
2020-01-29  Sergio Durigan Junior  <sergiodj@redhat.com>

	* infcmd.c (construct_inferior_arguments): Assert that
	'length' is greater than 0.

Change-Id: Ide8407cbedcb4921de1843a6a15bbcb7676c7d26
---
 gdb/infcmd.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/gdb/infcmd.c b/gdb/infcmd.c
index b44adca88d..f468c788b6 100644
--- a/gdb/infcmd.c
+++ b/gdb/infcmd.c
@@ -365,6 +365,11 @@ construct_inferior_arguments (int argc, char **argv)
 	  length += strlen (argv[i]) + 1;
 	}
 
+      /* argc should always be at least 1, but we double check this
+	 here.  This is also needed to silence -Werror-stringop
+	 warnings.  */
+      gdb_assert (length > 0);
+
       result = (char *) xmalloc (length);
       result[0] = '\0';
       for (i = 0; i < argc; ++i)
-- 
2.21.0