ext/gmp: gmp_pow fix FPE with large values.

even without sanitizers, it is reproducible but with the following

```
<?php
$g = gmp_init(256);
var_dump(gmp_pow($g, PHP_INT_MAX));
```

we get this

```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==286922==ERROR: AddressSanitizer: FPE on unknown address 0x03e8000460ca (pc 0x7faf6c69de5c bp 0x400000000000004 sp 0x7ffe9843c740 T0)
    #0 0x7faf6c69de5c in __pthread_kill_implementation nptl/pthread_kill.c:44
    #1 0x7faf6c649c81 in __GI_raise ../sysdeps/posix/raise.c:26
    php#2 0x7faf6db9386c in __gmp_exception (/lib/x86_64-linux-gnu/libgmp.so.10+0xd86c) (BuildId: 1af68a49fe041a5bb48a2915c3d47541f713bb38)
    php#3 0x7faf6db938d3 in __gmp_overflow_in_mpz (/lib/x86_64-linux-gnu/libgmp.so.10+0xd8d3) (BuildId: 1af68a49fe041a5bb48a2915c3d47541f713bb38)
    php#4 0x7faf6dbac95c in __gmpz_realloc (/lib/x86_64-linux-gnu/libgmp.so.10+0x2695c) (BuildId: 1af68a49fe041a5bb48a2915c3d47541f713bb38)
    php#5 0x7faf6dba9038 in __gmpz_n_pow_ui (/lib/x86_64-linux-gnu/libgmp.so.10+0x23038) (BuildId: 1af68a49fe041a5bb48a2915c3d47541f713bb38)
    php#6 0x5565ae1ccd9f in zif_gmp_pow /home/dcarlier/Contribs/php-src/ext/gmp/gmp.c:1286
    php#7 0x5565aee96ea9 in ZEND_DO_ICALL_SPEC_RETVAL_USED_HANDLER /home/dcarlier/Contribs/php-src/Zend/zend_vm_execute.h:1312
    php#8 0x5565af144320 in execute_ex /home/dcarlier/Contribs/php-src/Zend/zend_vm_execute.h:56075
    php#9 0x5565af160f07 in zend_execute /home/dcarlier/Contribs/php-src/Zend/zend_vm_execute.h:60439
    php#10 0x5565aed6fafe in zend_execute_scripts /home/dcarlier/Contribs/php-src/Zend/zend.c:1842
    php#11 0x5565aeae70a8 in php_execute_script /home/dcarlier/Contribs/php-src/main/main.c:2578
    php#12 0x5565af532f4e in do_cli /home/dcarlier/Contribs/php-src/sapi/cli/php_cli.c:964
    php#13 0x5565af535877 in main /home/dcarlier/Contribs/php-src/sapi/cli/php_cli.c:1334
    php#14 0x7faf6c633d67 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    php#15 0x7faf6c633e24 in __libc_start_main_impl ../csu/libc-start.c:360
    php#16 0x5565adc04040 in _start (/home/dcarlier/Contribs/php-src/sapi/cli/php+0x2604040) (BuildId: 949049955bdf8b7197390b1978a1dfc3ef6fdf38)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: FPE nptl/pthread_kill.c:44 in __pthread_kill_implementation
==286922==ABORTING
```

Author: devnexen

ext/gmp/gmp.c

@@ -1278,11 +1278,21 @@ ZEND_FUNCTION(gmp_pow)
 
 	if (Z_TYPE_P(base_arg) == IS_LONG && Z_LVAL_P(base_arg) >= 0) {
 		INIT_GMP_RETVAL(gmpnum_result);
+		if ((log10(Z_LVAL_P(base_arg)) * exp) > (double)ULONG_MAX) {
+			zend_value_error("base and exponent overflow");
+			RETURN_THROWS();
+		}
 		mpz_ui_pow_ui(gmpnum_result, Z_LVAL_P(base_arg), exp);
 	} else {
 		mpz_ptr gmpnum_base;
+		unsigned long gmpnum;
 		FETCH_GMP_ZVAL(gmpnum_base, base_arg, temp_base, 1);
 		INIT_GMP_RETVAL(gmpnum_result);
+		gmpnum = mpz_get_ui(gmpnum_base);
+		if ((log10(gmpnum) * exp) > (double)ULONG_MAX) {
+			zend_value_error("base and exponent overflow");
+			RETURN_THROWS();
+		}
 		mpz_pow_ui(gmpnum_result, gmpnum_base, exp);
 		FREE_GMP_TEMP(temp_base);
 	}