_R[15] access causes issues downstream

[danmatichuk]

Later tools can't translate the call to "uf_get_gpr" if the argument is 15, but the current definition of "_R[n]" doesn't immediately preclude this case (it guards with assertions, but those are currently discarded).

This yields an error on some binaries, i.e.

000100d8 <_start>:
   100d8:       e52db004        push    {fp}            ; (str fp, [sp, #-4]!)
   100dc:       e28db000        add     fp, sp, #0
   100e0:       e59f3054        ldr     r3, [pc, #84]   ; 1013c <_start+0x64>
   100e4:       e08f3003        add     r3, pc, r3
   100e8:       e59f2050        ldr     r2, [pc, #80]   ; 10140 <_start+0x68>
   100ec:       e7932002        ldr     r2, [r3, r2]
   100f0:       e5922000        ldr     r2, [r2]
   100f4:       e3520000        cmp     r2, #0
   100f8:       da00000b        ble     1012c <_start+0x54>
   100fc:       e59f203c        ldr     r2, [pc, #60]   ; 10140 <_start+0x68>
   10100:       e7932002        ldr     r2, [r3, r2]
   10104:       e5922000        ldr     r2, [r2]
   10108:       e3520000        cmp     r2, #0
   1010c:       da000006        ble     1012c <_start+0x54>
   10110:       e59f2028        ldr     r2, [pc, #40]   ; 10140 <_start+0x68>
   10114:       e7932002        ldr     r2, [r3, r2]
   10118:       e5922000        ldr     r2, [r2]
   1011c:       e2822001        add     r2, r2, #1
   10120:       e59f1018        ldr     r1, [pc, #24]   ; 10140 <_start+0x68>
   10124:       e7933001        ldr     r3, [r3, r1]
   10128:       e5832000        str     r2, [r3]
   1012c:       e1a00000        nop                     ; (mov r0, r0)
   10130:       e24bd000        sub     sp, fp, #0
   10134:       e49db004        pop     {fp}            ; (ldr fp, [sp], #4)
   10138:       e12fff1e        bx      lr
   1013c:       00010058        .word   0x00010058
   10140:       0000000c        .word   0x0000000c

The simple fix is to guard this function with a redundant check on n.