Fix GH-17503: Undefined float conversion in mb_convert_variables

Conversion of floating point to integer values is undefined if the
integral part of the float value cannot be represented by the integer
type.  We need to cater to that explicitly (in a manner similar to
`zend_dval_to_lval_cap()`).

ext/mbstring/mbstring.c

@@ -3092,7 +3092,8 @@ try_next_encoding:;
 	}
 
 	for (size_t i = 0; i < length; i++) {
-		array[i].demerits *= array[i].multiplier;
+		float demerits = array[i].demerits * array[i].multiplier;
+		array[i].demerits = demerits < (float) UINT64_MAX ? (uint64_t) demerits : UINT64_MAX;
 	}
 
 	return length;

ext/mbstring/tests/gh17503.phpt

@@ -0,0 +1,11 @@
+--TEST--
+GH-17503 (Undefined float conversion in mb_convert_variables)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+$a = array_fill(0, 500, "<blah>");
+var_dump(mb_convert_variables("ASCII", ["UTF-8", "UTF-16"], $a));
+?>
+--EXPECT--
+string(5) "UTF-8"