BUG/MINOR: cfgparse: don't increment linenum on incomplete lines

BUG/MINOR: cfgparse: don't increment linenum on incomplete lines

When fgets() returns an incomplete line we must not increment linenum
otherwise line numbers become incorrect. This may happen when parsing
files with extremely long lines which require a realloc().

The bug has been present since unbounded line length was supported, so
the fix should be backported to older branches.

(cherry picked from commit 40cb26f6ec8cf9d8ba54706d95df4cfed3a7b332)
[wt: adjusted context wrt missing fatal++]
Signed-off-by: Willy Tarreau <w@1wt.eu>