gdb: Fix reading of partial symtabs in dbxread.c

After commit 9675da25 ("Use unrelocated_addr in minimal symbols"),
aarch64-linux started failing gdb.asm/asm-source.exp:

Running /home/thiago.bauermann/src/binutils-gdb/gdb/testsuite/gdb.asm/asm-source.exp ...
PASS: gdb.asm/asm-source.exp: f at main
PASS: gdb.asm/asm-source.exp: n at main
PASS: gdb.asm/asm-source.exp: next over macro
FAIL: gdb.asm/asm-source.exp: step into foo2
PASS: gdb.asm/asm-source.exp: info target
PASS: gdb.asm/asm-source.exp: info symbol
PASS: gdb.asm/asm-source.exp: list
PASS: gdb.asm/asm-source.exp: search
FAIL: gdb.asm/asm-source.exp: f in foo2
FAIL: gdb.asm/asm-source.exp: n in foo2 (the program exited)
FAIL: gdb.asm/asm-source.exp: bt ALL in foo2
FAIL: gdb.asm/asm-source.exp: bt 2 in foo2
PASS: gdb.asm/asm-source.exp: s 2
PASS: gdb.asm/asm-source.exp: n 2
FAIL: gdb.asm/asm-source.exp: bt 3 in foo3
PASS: gdb.asm/asm-source.exp: info source asmsrc1.s
FAIL: gdb.asm/asm-source.exp: finish from foo3 (the program is no longer running)
FAIL: gdb.asm/asm-source.exp: info source asmsrc2.s
PASS: gdb.asm/asm-source.exp: info sources
FAIL: gdb.asm/asm-source.exp: info line
FAIL: gdb.asm/asm-source.exp: next over foo3 (the program is no longer running)
FAIL: gdb.asm/asm-source.exp: return from foo2
PASS: gdb.asm/asm-source.exp: look at global variable
PASS: gdb.asm/asm-source.exp: x/i &globalvar
PASS: gdb.asm/asm-source.exp: disassem &globalvar, (int *) &globalvar+1
PASS: gdb.asm/asm-source.exp: look at static variable
PASS: gdb.asm/asm-source.exp: x/i &staticvar
PASS: gdb.asm/asm-source.exp: disassem &staticvar, (int *) &staticvar+1
PASS: gdb.asm/asm-source.exp: look at static function

The problem is simple: a pair of parentheses was removed from the
expression calculating text_end and thus text_size was only added if
lowest_text_address wasn't equal to -1.

This patch restores the previous behaviour and fixes the testcase.
Tested on native aarch64-linux.

Reviewed-By: Tom Tromey <tom@tromey.com>