Fix #111642: VSE glow code can overflow with small proxies

With such small proxy sizes (combined with a small blur radius), the
kernels `halfWidth` can get zero, which leads to a memory allocation of
same zero size and writing to that memory leads to overflow/crashes/can
only go downhill from there.

Now early out in such cases [which leads to
slightly different output -- well if the "buggy" output survives and
does not crash that is].

(alternatively we could just prevent the overflow and still let do
`RVBlurBitmap2_float` do stuff that it really shouldnt imo, see first version of the PR)

Pull Request: https://projects.blender.org/blender/blender/pulls/111660

source/blender/sequencer/intern/effects.cc

@@ -1980,6 +1980,12 @@ static void RVBlurBitmap2_float(float *map, int width, int height, float blur, i
     return;
   }
 
+  /* If result would be no blurring, early out. */
+  halfWidth = ((quality + 1) * blur);
+  if (halfWidth == 0) {
+    return;
+  }
+
   /* Allocate memory for the temp-map and the blur filter matrix. */
   temp = static_cast<float *>(MEM_mallocN(sizeof(float[4]) * width * height, "blurbitmaptemp"));
   if (!temp) {
@@ -1987,7 +1993,6 @@ static void RVBlurBitmap2_float(float *map, int width, int height, float blur, i
   }
 
   /* Allocate memory for the filter elements */
-  halfWidth = ((quality + 1) * blur);
   filter = (float *)MEM_mallocN(sizeof(float) * halfWidth * 2, "blurbitmapfilter");
   if (!filter) {
     MEM_freeN(temp);