Fix mbt L null row size consideration

[fspindle]

Avoid stacking L matrix when the number of features is 0
Allows to fix the following issue for example occuring in testGenericTracker.cpp when GSL is used.

Occurs on the followings ci machines: Fedora 25 with GSL 2.1 and centos 7.2 with GSL 1.15

./testGenericTracker
trackerType_image: 1
useScanline: 0
use_depth: 0
use_mask: 0
use_color_image: 0
Load config file for camera 1: /builds/visp-ws/visp-images/mbt-depth/Castle-simu/Config/chateau.xml Load config file for camera 2: /builds/visp-ws/visp-images/mbt-depth/Castle-simu/Config/chateau_depth.xml
 *********** Parsing XML for ME projection error ************
projection_error : sample_step : 10 (default)
projection_error : kernel_size : 5x5 (default)
 *********** Parsing XML for Edge Model-Based Tracker ************
me : mask : size : 5
me : mask : nb_mask : 180
me : range : tracking : 8
me : range : init range : 0 (default)
me : sample : sample_step : 5
me : contrast : threshold type 1
me : contrast : threshold 5
me : contrast : min threshold -1 (default)
me : contrast : threshold margin ratio -1 (default) me : contrast : mu1 0.5
me : contrast : mu2 0.5
camera : u0 : 320
camera : v0 : 240
camera : px : 700
camera : py : 700
face : Angle Appear : 85
face : Angle Disappear : 89
face : Near Clipping : 0.01
face : Far Clipping : 2
face : Fov Clipping : True
lod : use lod : 0 (default)
lod : min line length threshold : 50 (default)
lod : min polygon area threshold : 2500 (default)
 *********** Parsing XML for ME projection error ************
projection_error : sample_step : 10 (default)
projection_error : kernel_size : 5x5 (default)
 *********** Parsing XML for Depth Dense Model-Based Tracker ************
depth dense : sampling_step : step_X : 4
depth dense : sampling_step : step_Y : 4
camera : u0 : 320
camera : v0 : 240
camera : px : 700
camera : py : 700
face : Angle Appear : 85
face : Angle Disappear : 89
face : Near Clipping : 0.01
face : Far Clipping : 2
face : Fov Clipping : True
lod : use lod : 0 (default)
lod : min line length threshold : 50 (default)
lod : min polygon area threshold : 2500 (default)
> 14 points
> 0 lines
> 0 polygon lines
> 5 polygon points
> 0 cylinders
> 0 circles
> 14 points
> 0 lines
> 0 polygon lines
> 5 polygon points
> 0 cylinders
> 0 circles
> 10 points
> 0 lines
> 0 polygon lines
> 4 polygon points
> 0 cylinders
> 10 points
> 0 lines
> 0 polygon lines
> 4 polygon points
> 0 cylinders
gsl: view_source.c:63: ERROR: matrix dimension n1 must be positive integer Default GSL error handler invoked.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 33.21%. Comparing base (6301603) to head (aef6ad8).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
modules/tracker/mbt/src/vpMbGenericTracker.cpp	75.00%	0 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (6301603) and HEAD (aef6ad8). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (6301603)	HEAD (aef6ad8)
	3	2

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1855       +/-   ##
===========================================
- Coverage   47.82%   33.21%   -14.62%     
===========================================
  Files         532      465       -67     
  Lines       68920    65999     -2921     
  Branches    32182    28708     -3474     
===========================================
- Hits        32963    21919    -11044     
- Misses      31902    33543     +1641     
- Partials     4055    10537     +6482     

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