Bug 788884 - Fix compiler warning and potential integer underflow [r=bsmedberg]

b2g/app/nsBrowserApp.cpp

@@ -164,7 +164,7 @@ int main(int argc, char* argv[])
   }
 
   char *lastSlash = strrchr(exePath, XPCOM_FILE_PATH_SEPARATOR[0]);
-  if (!lastSlash || (lastSlash - exePath > MAXPATHLEN - sizeof(XPCOM_DLL) - 1))
+  if (!lastSlash || ((lastSlash - exePath) + sizeof(XPCOM_DLL) + 1 > MAXPATHLEN))
     return 255;
 
   strcpy(++lastSlash, XPCOM_DLL);