Fix #78878: Buffer underflow in bc_shift_addsub

We must not rely on `isdigit()` to detect digits, since we only support
decimal ASCII digits in the following processing.

(cherry picked from commit eb23c60)

ext/bcmath/libbcmath/src/str2num.c

@@ -57,9 +57,9 @@ bc_str2num (bc_num *num, char *str, int scale)
   zero_int = FALSE;
   if ( (*ptr == '+') || (*ptr == '-'))  ptr++;  /* Sign */
   while (*ptr == '0') ptr++;			/* Skip leading zeros. */
-  while (isdigit((int)*ptr)) ptr++, digits++;	/* digits */
+  while (*ptr >= '0' && *ptr <= '9') ptr++, digits++;	/* digits */
   if (*ptr == '.') ptr++;			/* decimal point */
-  while (isdigit((int)*ptr)) ptr++, strscale++;	/* digits */
+  while (*ptr >= '0' && *ptr <= '9') ptr++, strscale++;	/* digits */
   if ((*ptr != '\0') || (digits+strscale == 0))
     {
       *num = bc_copy_num (BCG(_zero_));

ext/bcmath/tests/bug78878.phpt

@@ -0,0 +1,13 @@
+--TEST--
+Bug #78878 (Buffer underflow in bc_shift_addsub)
+--SKIPIF--
+<?php
+if (!extension_loaded('bcmath')) die('skip bcmath extension not available');
+?>
+--FILE--
+<?php
+print @bcmul("\xB26483605105519922841849335928742092", bcpowmod(2, 65535, -4e-4));
+?>
+--EXPECT--
+bc math warning: non-zero scale in modulus
+0