stb_image.h: apply mainstream PR/1736

Fix root-cause of CVE-2021-45340 : dereference of NULL ptr.
Patch authored by Henner Zeller <[email protected]>
Mainstream pull request: nothings/stb#1736

(cherry picked from commit d345579)

Author: sezero

src/stb_image.h

@@ -100,7 +100,7 @@ RECENT REVISION HISTORY:
  Bug & warning fixes
     Marc LeBlanc            David Woo          Guillaume George     Martins Mozeiko
     Christpher Lloyd        Jerry Jansson      Joseph Thomson       Blazej Dariusz Roszkowski
-    Phil Jordan                                Dave Moore           Roy Eltham
+    Phil Jordan             Henner Zeller      Dave Moore           Roy Eltham
     Hayaki Saito            Nathan Reed        Won Chun
     Luke Graham             Johan Duparc       Nick Verigakis       the Horde3D community
     Thomas Ruf              Ronny Chevalier                         github:rlyeh
@@ -1895,6 +1895,7 @@ static unsigned char *stbi__convert_format(unsigned char *data, int img_n, int r
    int i,j;
    unsigned char *good;
 
+   if (data == NULL) return data;
    if (req_comp == img_n) return data;
    STBI_ASSERT(req_comp >= 1 && req_comp <= 4);