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()`).

Closes GH-17689.

NEWS

@@ -12,6 +12,10 @@ PHP                                                                        NEWS
     zend.exception_ignore_args=1 into account). (timwolla)
   . Fix fallback paths in fast_long_{add,sub}_function. (nielsdos)
 
+- MBString:
+  . Fixed bug GH-17503 (Undefined float conversion in mb_convert_variables).
+    (cmb)
+
 - Opcache:
   . Fixed bug GH-17654 (Multiple classes using same trait causes function
     JIT crash). (nielsdos)

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;
+		double demerits = array[i].demerits * (double) array[i].multiplier;
+		array[i].demerits = demerits < (double) 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"