Fix segfault when using an unknown identifier

[DimitryAndric]

For example, the sin() function is not yet supported, but when trying sin(pi) it says (with UndefinedBehaviorSanitizer enabled):

Unknown identifier 'sin(pi)'eval.c:452:23: runtime error: member access within misaligned address 0x1a9f07e9f100011f for type 'Expression' (aka 'struct Expression'), which requires 8 byte alignment
0x1a9f07e9f100011f: note: pointer points here
<memory cannot be printed>
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior eval.c:452:23 in
eval.c:452:23: runtime error: load of misaligned address 0x1a9f07e9f100011f for type 'enum (anonymous enum at eval.c:11:3)', which requires 8 byte alignment
0x1a9f07e9f100011f: note: pointer points here
<memory cannot be printed>
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior eval.c:452:23 in
UndefinedBehaviorSanitizer:DEADLYSIGNAL
==55692==ERROR: UndefinedBehaviorSanitizer: SEGV on unknown address 0xffff87e9f100011f (pc 0x0001001ed700 bp 0x0001001ed6fc sp 0x00016fc16fa0 T3060488)
==55692==The signal is caused by a UNKNOWN memory access.
    #0 0x1001ed700 in parse_verify eval.c:452

==55692==Register values:
 x[0] = 0x000000016fc16ea0   x[1] = 0x0000000000000000   x[2] = 0x0000000000000000   x[3] = 0x00000001012003d0
 x[4] = 0x6a6cb03abcebc041   x[5] = 0x0000000000000000   x[6] = 0x0000000000000000   x[7] = 0x0000000000000000
 x[8] = 0x1a9f07e9f100011f   x[9] = 0x000000000000d810  x[10] = 0x00000000000c6230  x[11] = 0x0000000000000001
x[12] = 0x0000000000000000  x[13] = 0x00000001005b7153  x[14] = 0x0000000000000000  x[15] = 0x0000000000000000
x[16] = 0x0000000000000049  x[17] = 0x0000000100410058  x[18] = 0x0000000000000000  x[19] = 0x0000000000000000
x[20] = 0x0000000000000000  x[21] = 0x0000000000000000  x[22] = 0x0000000000000000  x[23] = 0x0000000000000000
x[24] = 0x0000000000000000  x[25] = 0x0000000000000000  x[26] = 0x0000000000000000  x[27] = 0x0000000000000000
x[28] = 0x000000016fc177c8     fp = 0x000000016fc170a0     lr = 0x00000001001ed6fc     sp = 0x000000016fc16fa0
UndefinedBehaviorSanitizer can not provide additional info.
SUMMARY: UndefinedBehaviorSanitizer: SEGV eval.c:452 in parse_verify
==55692==ABORTING

This turns out to be due to parse_primary() returning true for unknown identifiers. Return false instead, and add a newline to the error message, similar to the other errors in this function.