Fix computing string length with indirect symbols from symbols files.

[rolfbjarne]

Indirect symbols are copied twice to the 'p' variable, this is the second time:

cctools/misc/strip.c

Lines 4217 to 4228 in cbe977a

	if((n_type & N_TYPE) == N_INDR){
	if(n_value != 0){
	strcpy(p, strings + n_value);
	if(object->mh != NULL)
	new_symbols[inew_syms].n_value =
	(uint32_t)(p - new_strings);
	else
	new_symbols64[inew_syms].n_value =
	(uint32_t)(p - new_strings);
	p += strlen(p) + 1;
	}
	}

This means we need to add the size of the indirect symbol to the size of the 'p' memory contents twice.

This seems to already be happening here:

cctools/misc/strip.c

Lines 3820 to 3823 in cbe977a

	if((n_type & N_TYPE) == N_INDR){
	len = strlen(strings + n_value) + 1;
	new_ext_strsize += len;
	}

and:

cctools/misc/strip.c

Lines 3845 to 3848 in cbe977a

	if((n_type & N_TYPE) == N_INDR){
	len = strlen(strings + n_value) + 1;
	new_ext_strsize += len;
	}

but one scenario (symbols files) was missing.

Ref: FB13327641 (contains test case).
Ref: dotnet/macios#19157