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.

src/cfgparse.c

@@ -1903,12 +1903,14 @@ next_line:
 					 file, linenum);
 				err_code |= ERR_ALERT | ERR_FATAL;
 				fatal++;
+				linenum--;
 				continue;
 			}
 
 			readbytes = linesize - 1;
 			linesize = newlinesize;
 			thisline = newline;
+			linenum--;
 			continue;
 		}