fix problem with 0-length string

message:  Non-initialized "ptr" is passed to "lua_pushlstring" as const

When input length is zero the transformation loop does not run,
leaving buffer contents uninitialized. Although lua_pushlstring with
length 0 would not read from ptr, some static analyzers flag this as
use of uninitialized data. Provide explicit fast-path for n==0 to
make intent clear and silence warnings (CERT_C-EXP33-a-1).

Author: maxim-laschinsky

src/lcurl.c

@@ -230,6 +230,11 @@ static int push_upper(lua_State *L, const char *str){
   size_t i, n = strlen(str);
   char *ptr = (n < sizeof(buffer))?&buffer[0]:malloc(n + 1);
   if (!ptr) return 1;
+  if (n == 0) {
+    lua_pushlstring(L, "", 0);
+    if(ptr != &buffer[0]) free(ptr);
+    return 0;
+  }
   for(i = 0; i < n; ++i){
     if( (str[i] > 96 ) && (str[i] < 123) ) ptr[i] = str[i] - 'a' + 'A';
     else ptr[i] = str[i];