Merge branch 'master' into fix-1291-xgrid-segfault

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index fe9ac1c215a1ec749724bd181b6dd6cfef4e8149,7a7fd94e8e7b63926de354d0908f4bd444b33a64..442d561c18b8edc7e7cf03aa593266a35571d2fe
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--- 2/CHANGES
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@@@ -3,14 -3,11 +3,19 @@@ RRDtool - master ..
  ====================
  Bugfixes
  --------
 +* Fix segfault when `--x-grid` is given without a trailing date-format
 +  field. The `%n` conversion in `sscanf()` is not counted in its return
 +  value, so `stroff` remained uninitialized when the input ended before
 +  the final `:` separator; the subsequent `stroff != 0` guard read
 +  garbage and could branch into `strdup()` with a bogus offset. Fixed by
 +  initialising `stroff = 0` so a partial match safely falls through to
 +  the existing "invalid x-grid format" error path. Issue #1291
 +  reported by @anvilvapre, fixed by @oetiker
+ * libdbi /derive now returns 0 instead of NaN when the underlying
+   counter is constant (zero delta). Previously the strict `d_value > 0`
+   guard skipped zero deltas the same way it skips negative ones (counter
+   resets). Changed to `d_value >= 0`. Issue #550 reported by
+   @bitionaire, fix by @oetiker.
  * Fix a 1-byte heap buffer overflow in `checkUnusedValues()` that
    triggered `*** buffer overflow detected ***` SIGABRT under glibc
    fortified `strcat()` when reporting unused graph arguments. The